working examples 4 and 5

canvas/Cargo.toml

@@ -16,7 +16,7 @@ crossterm = { workspace = true, optional = true }
 anyhow.workspace = true
 tokio = { workspace = true, optional = true }
 toml = { workspace = true }
-serde = { workspace = true }
+serde.workspace = true
 unicode-width.workspace = true
 thiserror = { workspace = true }
 

canvas/examples/validation_2.rs

@@ -0,0 +1,647 @@
+// examples/validation_2.rs
+//! Advanced TUI Example demonstrating complex pattern filtering edge cases
+//!
+//! This example showcases the full potential of the pattern validation system
+//! with creative real-world scenarios and edge cases.
+//!
+//! Run with: cargo run --example validation_advanced_patterns --features "validation,gui"
+
+// REQUIRE validation and gui features
+#[cfg(not(all(feature = "validation", feature = "gui")))]
+compile_error!(
+    "This example requires the 'validation' and 'gui' features. \
+     Run with: cargo run --example validation_advanced_patterns --features \"validation,gui\""
+);
+
+use std::io;
+use std::sync::Arc;
+use canvas::ValidationResult;
+use crossterm::{
+    event::{
+        self, DisableMouseCapture, EnableMouseCapture, Event, KeyCode, KeyModifiers,
+    },
+    execute,
+    terminal::{
+        disable_raw_mode, enable_raw_mode, EnterAlternateScreen, LeaveAlternateScreen,
+    },
+};
+use ratatui::{
+    backend::{Backend, CrosstermBackend},
+    layout::{Constraint, Direction, Layout, Rect},
+    style::{Color, Style},
+    text::{Line, Span},
+    widgets::{Block, Borders, Paragraph, Wrap},
+    Frame, Terminal,
+};
+
+use canvas::{
+    canvas::{
+        gui::render_canvas_default,
+        modes::AppMode,
+    },
+    DataProvider, FormEditor,
+    ValidationConfig, ValidationConfigBuilder, PatternFilters, PositionFilter, PositionRange, CharacterFilter,
+};
+
+// Enhanced FormEditor wrapper (keeping the same structure as before)
+struct AdvancedPatternFormEditor<D: DataProvider> {
+    editor: FormEditor<D>,
+    debug_message: String,
+    command_buffer: String,
+    validation_enabled: bool,
+    field_switch_blocked: bool,
+    block_reason: Option<String>,
+}
+
+impl<D: DataProvider> AdvancedPatternFormEditor<D> {
+    fn new(data_provider: D) -> Self {
+        let mut editor = FormEditor::new(data_provider);
+        editor.set_validation_enabled(true);
+
+        Self {
+            editor,
+            debug_message: "🚀 Advanced Pattern Validation - Showcasing edge cases and complex patterns!".to_string(),
+            command_buffer: String::new(),
+            validation_enabled: true,
+            field_switch_blocked: false,
+            block_reason: None,
+        }
+    }
+
+    // ... (keeping all the same methods as before for brevity)
+    // [All the previous methods: clear_command_buffer, add_to_command_buffer, etc.]
+    
+    fn clear_command_buffer(&mut self) { self.command_buffer.clear(); }
+    fn add_to_command_buffer(&mut self, ch: char) { self.command_buffer.push(ch); }
+    fn get_command_buffer(&self) -> &str { &self.command_buffer }
+    fn has_pending_command(&self) -> bool { !self.command_buffer.is_empty() }
+    
+    fn toggle_validation(&mut self) {
+        self.validation_enabled = !self.validation_enabled;
+        self.editor.set_validation_enabled(self.validation_enabled);
+        if self.validation_enabled {
+            self.debug_message = "✅ Advanced Pattern Validation ENABLED".to_string();
+        } else {
+            self.debug_message = "❌ Advanced Pattern Validation DISABLED".to_string();
+        }
+    }
+
+    fn move_left(&mut self) { self.editor.move_left(); self.field_switch_blocked = false; self.block_reason = None; }
+    fn move_right(&mut self) { self.editor.move_right(); self.field_switch_blocked = false; self.block_reason = None; }
+    
+    fn move_up(&mut self) {
+        match self.editor.move_up() {
+            Ok(()) => { self.update_field_validation_status(); self.field_switch_blocked = false; self.block_reason = None; }
+            Err(e) => { self.field_switch_blocked = true; self.block_reason = Some(e.to_string()); self.debug_message = format!("🚫 Field switch blocked: {}", e); }
+        }
+    }
+    
+    fn move_down(&mut self) {
+        match self.editor.move_down() {
+            Ok(()) => { self.update_field_validation_status(); self.field_switch_blocked = false; self.block_reason = None; }
+            Err(e) => { self.field_switch_blocked = true; self.block_reason = Some(e.to_string()); self.debug_message = format!("🚫 Field switch blocked: {}", e); }
+        }
+    }
+
+    fn move_line_start(&mut self) { self.editor.move_line_start(); }
+    fn move_line_end(&mut self) { self.editor.move_line_end(); }
+    
+    fn enter_edit_mode(&mut self) {
+        self.editor.enter_edit_mode();
+        self.debug_message = "✏️  INSERT MODE - Testing advanced pattern validation".to_string();
+    }
+    
+    fn enter_append_mode(&mut self) {
+        self.editor.enter_append_mode();
+        self.debug_message = "✏️  INSERT (append) - Advanced patterns active".to_string();
+    }
+    
+    fn exit_edit_mode(&mut self) {
+        self.editor.exit_edit_mode();
+        self.debug_message = "🔒 NORMAL MODE".to_string();
+        self.update_field_validation_status();
+    }
+
+    fn insert_char(&mut self, ch: char) -> anyhow::Result<()> {
+        let result = self.editor.insert_char(ch);
+        if result.is_ok() {
+            if let Some(validation_result) = self.editor.current_field_validation() {
+                match validation_result {
+                    ValidationResult::Valid => { self.debug_message = "✅ Character accepted".to_string(); }
+                    ValidationResult::Warning { message } => { self.debug_message = format!("⚠️  Warning: {}", message); }
+                    ValidationResult::Error { message } => { self.debug_message = format!("❌ Pattern violation: {}", message); }
+                }
+            }
+        }
+        Ok(result?)
+    }
+
+    fn delete_backward(&mut self) -> anyhow::Result<()> {
+        let result = self.editor.delete_backward();
+        if result.is_ok() { self.debug_message = "⌫ Character deleted".to_string(); }
+        Ok(result?)
+    }
+
+    fn delete_forward(&mut self) -> anyhow::Result<()> {
+        let result = self.editor.delete_forward();
+        if result.is_ok() { self.debug_message = "⌦ Character deleted".to_string(); }
+        Ok(result?)
+    }
+
+    // Delegate methods
+    fn current_field(&self) -> usize { self.editor.current_field() }
+    fn cursor_position(&self) -> usize { self.editor.cursor_position() }
+    fn mode(&self) -> AppMode { self.editor.mode() }
+    fn current_text(&self) -> &str { self.editor.current_text() }
+    fn data_provider(&self) -> &D { self.editor.data_provider() }
+    fn ui_state(&self) -> &canvas::EditorState { self.editor.ui_state() }
+    fn set_mode(&mut self, mode: AppMode) { self.editor.set_mode(mode); }
+
+    fn next_field(&mut self) {
+        match self.editor.next_field() {
+            Ok(()) => { self.update_field_validation_status(); self.field_switch_blocked = false; self.block_reason = None; }
+            Err(e) => { self.field_switch_blocked = true; self.block_reason = Some(e.to_string()); self.debug_message = format!("🚫 Cannot move to next field: {}", e); }
+        }
+    }
+
+    fn prev_field(&mut self) {
+        match self.editor.prev_field() {
+            Ok(()) => { self.update_field_validation_status(); self.field_switch_blocked = false; self.block_reason = None; }
+            Err(e) => { self.field_switch_blocked = true; self.block_reason = Some(e.to_string()); self.debug_message = format!("🚫 Cannot move to previous field: {}", e); }
+        }
+    }
+
+    fn set_debug_message(&mut self, msg: String) { self.debug_message = msg; }
+    fn debug_message(&self) -> &str { &self.debug_message }
+
+    fn update_field_validation_status(&mut self) {
+        if !self.validation_enabled { return; }
+        if let Some(result) = self.editor.current_field_validation() {
+            match result {
+                ValidationResult::Valid => { self.debug_message = format!("Field {}: ✅ Pattern valid", self.editor.current_field() + 1); }
+                ValidationResult::Warning { message } => { self.debug_message = format!("Field {}: ⚠️  {}", self.editor.current_field() + 1, message); }
+                ValidationResult::Error { message } => { self.debug_message = format!("Field {}: ❌ {}", self.editor.current_field() + 1, message); }
+            }
+        }
+    }
+
+    fn get_validation_status(&self) -> String {
+        if !self.validation_enabled { return "❌ DISABLED".to_string(); }
+        if self.field_switch_blocked { return "🚫 SWITCH BLOCKED".to_string(); }
+        let summary = self.editor.validation_summary();
+        if summary.has_errors() { format!("❌ {} ERRORS", summary.error_fields) }
+        else if summary.has_warnings() { format!("⚠️  {} WARNINGS", summary.warning_fields) }
+        else if summary.validated_fields > 0 { format!("✅ {} VALID", summary.valid_fields) }
+        else { "🔍 READY".to_string() }
+    }
+}
+
+// Advanced demo form with creative and edge-case-heavy validation patterns
+struct AdvancedPatternData {
+    fields: Vec<(String, String)>,
+}
+
+impl AdvancedPatternData {
+    fn new() -> Self {
+        Self {
+            fields: vec![
+                ("🕐 Time (HH:MM) - 24hr format".to_string(), "".to_string()),
+                ("🎨 Hex Color (#RRGGBB) - Web colors".to_string(), "".to_string()),
+                ("🌐 IPv4 (XXX.XXX.XXX.XXX) - Network address".to_string(), "".to_string()),
+                ("🏷️  Product Code (ABC-123-XYZ) - Mixed format".to_string(), "".to_string()),
+                ("📅 Date Code (2024W15) - Year + Week".to_string(), "".to_string()),
+                ("🔢 Binary (101010) - Only 0s and 1s".to_string(), "".to_string()),
+                ("🎯 Complex ID (A1-B2C-3D4E) - Multi-rule".to_string(), "".to_string()),
+                ("🚀 Custom Pattern - Advanced logic".to_string(), "".to_string()),
+            ],
+        }
+    }
+}
+
+impl DataProvider for AdvancedPatternData {
+    fn field_count(&self) -> usize { self.fields.len() }
+    fn field_name(&self, index: usize) -> &str { &self.fields[index].0 }
+    fn field_value(&self, index: usize) -> &str { &self.fields[index].1 }
+    fn set_field_value(&mut self, index: usize, value: String) { self.fields[index].1 = value; }
+
+    fn validation_config(&self, field_index: usize) -> Option<ValidationConfig> {
+        match field_index {
+            0 => {
+                // 🕐 Time (HH:MM) - Hours 00-23, Minutes 00-59
+                // This showcases: Multiple position ranges, exact character matching, custom validation
+                let time_pattern = PatternFilters::new()
+                    .add_filter(PositionFilter::new(
+                        PositionRange::Multiple(vec![0, 1, 3, 4]), // Hours and minutes positions
+                        CharacterFilter::Numeric,
+                    ))
+                    .add_filter(PositionFilter::new(
+                        PositionRange::Single(2), // Colon separator
+                        CharacterFilter::Exact(':'),
+                    ));
+
+                Some(ValidationConfigBuilder::new()
+                    .with_pattern_filters(time_pattern)
+                    .with_max_length(5) // HH:MM = 5 characters
+                    .build())
+            }
+            1 => {
+                // 🎨 Hex Color (#RRGGBB) - Web color format
+                // This showcases: OneOf filter with hex digits, exact character at start
+                let hex_digits = vec!['0','1','2','3','4','5','6','7','8','9','A','B','C','D','E','F','a','b','c','d','e','f'];
+                let hex_color_pattern = PatternFilters::new()
+                    .add_filter(PositionFilter::new(
+                        PositionRange::Single(0), // Hash symbol
+                        CharacterFilter::Exact('#'),
+                    ))
+                    .add_filter(PositionFilter::new(
+                        PositionRange::Range(1, 6), // 6 hex digits for RGB
+                        CharacterFilter::OneOf(hex_digits),
+                    ));
+
+                Some(ValidationConfigBuilder::new()
+                    .with_pattern_filters(hex_color_pattern)
+                    .with_max_length(7) // #RRGGBB = 7 characters
+                    .build())
+            }
+            2 => {
+                // 🌐 IPv4 Address (XXX.XXX.XXX.XXX) - Network address
+                // This showcases: Complex pattern with dots at specific positions
+                let ipv4_pattern = PatternFilters::new()
+                    .add_filter(PositionFilter::new(
+                        PositionRange::Multiple(vec![3, 7, 11]), // Dots at specific positions
+                        CharacterFilter::Exact('.'),
+                    ))
+                    .add_filter(PositionFilter::new(
+                        PositionRange::Multiple(vec![0,1,2,4,5,6,8,9,10,12,13,14]), // Number positions
+                        CharacterFilter::Numeric,
+                    ));
+
+                Some(ValidationConfigBuilder::new()
+                    .with_pattern_filters(ipv4_pattern)
+                    .with_max_length(15) // XXX.XXX.XXX.XXX = up to 15 chars
+                    .build())
+            }
+            3 => {
+                // 🏷️ Product Code (ABC-123-XYZ) - Mixed format sections
+                // This showcases: Different rules for different sections
+                let product_code_pattern = PatternFilters::new()
+                    .add_filter(PositionFilter::new(
+                        PositionRange::Range(0, 2), // First 3 positions: letters
+                        CharacterFilter::Alphabetic,
+                    ))
+                    .add_filter(PositionFilter::new(
+                        PositionRange::Multiple(vec![3, 7]), // Dashes
+                        CharacterFilter::Exact('-'),
+                    ))
+                    .add_filter(PositionFilter::new(
+                        PositionRange::Range(4, 6), // Middle 3 positions: numbers
+                        CharacterFilter::Numeric,
+                    ))
+                    .add_filter(PositionFilter::new(
+                        PositionRange::Range(8, 10), // Last 3 positions: letters
+                        CharacterFilter::Alphabetic,
+                    ));
+
+                Some(ValidationConfigBuilder::new()
+                    .with_pattern_filters(product_code_pattern)
+                    .with_max_length(11) // ABC-123-XYZ = 11 characters
+                    .build())
+            }
+            4 => {
+                // 📅 Date Code (2024W15) - Year + Week format
+                // This showcases: From position filtering and mixed patterns
+                let date_code_pattern = PatternFilters::new()
+                    .add_filter(PositionFilter::new(
+                        PositionRange::Range(0, 3), // Year: 4 digits
+                        CharacterFilter::Numeric,
+                    ))
+                    .add_filter(PositionFilter::new(
+                        PositionRange::Single(4), // Week indicator
+                        CharacterFilter::Exact('W'),
+                    ))
+                    .add_filter(PositionFilter::new(
+                        PositionRange::From(5), // Week number: rest are digits
+                        CharacterFilter::Numeric,
+                    ));
+
+                Some(ValidationConfigBuilder::new()
+                    .with_pattern_filters(date_code_pattern)
+                    .with_max_length(7) // 2024W15 = 7 characters
+                    .build())
+            }
+            5 => {
+                // 🔢 Binary (101010) - Only 0s and 1s
+                // This showcases: OneOf filter with limited character set
+                let binary_pattern = PatternFilters::new()
+                    .add_filter(PositionFilter::new(
+                        PositionRange::From(0), // All positions
+                        CharacterFilter::OneOf(vec!['0', '1']),
+                    ));
+
+                Some(ValidationConfigBuilder::new()
+                    .with_pattern_filters(binary_pattern)
+                    .with_max_length(16) // Allow up to 16 binary digits
+                    .build())
+            }
+            6 => {
+                // 🎯 Complex ID (A1-B2C-3D4E) - Multiple overlapping rules
+                // This showcases: Complex overlapping patterns and edge cases
+                let complex_id_pattern = PatternFilters::new()
+                    .add_filter(PositionFilter::new(
+                        PositionRange::Multiple(vec![0, 3, 6, 8]), // Letter positions
+                        CharacterFilter::Alphabetic,
+                    ))
+                    .add_filter(PositionFilter::new(
+                        PositionRange::Multiple(vec![1, 4, 7, 9]), // Number positions
+                        CharacterFilter::Numeric,
+                    ))
+                    .add_filter(PositionFilter::new(
+                        PositionRange::Multiple(vec![2, 5]), // Dashes
+                        CharacterFilter::Exact('-'),
+                    ))
+                    .add_filter(PositionFilter::new(
+                        PositionRange::Single(5), // Special case: override dash with letter C
+                        CharacterFilter::Alphabetic, // This creates an interesting edge case
+                    ));
+
+                Some(ValidationConfigBuilder::new()
+                    .with_pattern_filters(complex_id_pattern)
+                    .with_max_length(10) // A1-B2C-3D4E = 10 characters
+                    .build())
+            }
+            7 => {
+                // 🚀 Custom Pattern - Advanced logic with custom function
+                // This showcases: Custom validation function for complex rules
+                let custom_pattern = PatternFilters::new()
+                    .add_filter(PositionFilter::new(
+                        PositionRange::From(0),
+                        CharacterFilter::Custom(Arc::new(|c| {
+                            // Advanced rule: Alternating vowels and consonants!
+                            // Even positions (0,2,4...): vowels (a,e,i,o,u)
+                            // Odd positions (1,3,5...): consonants
+                            let vowels = ['a', 'e', 'i', 'o', 'u', 'A', 'E', 'I', 'O', 'U'];
+                            
+                            // For demo purposes, we'll just accept alphabetic characters
+                            // In real usage, you'd implement the alternating logic based on position
+                            c.is_alphabetic()
+                        })),
+                    ));
+
+                Some(ValidationConfigBuilder::new()
+                    .with_pattern_filters(custom_pattern)
+                    .with_max_length(12) // Allow up to 12 characters
+                    .build())
+            }
+            _ => None,
+        }
+    }
+}
+
+// Key handling (same structure as before)
+fn handle_key_press(
+    key: KeyCode,
+    modifiers: KeyModifiers,
+    editor: &mut AdvancedPatternFormEditor<AdvancedPatternData>,
+) -> anyhow::Result<bool> {
+    let mode = editor.mode();
+
+    // Quit handling
+    if (key == KeyCode::Char('q') && modifiers.contains(KeyModifiers::CONTROL))
+        || (key == KeyCode::Char('c') && modifiers.contains(KeyModifiers::CONTROL))
+        || key == KeyCode::F(10)
+    {
+        return Ok(false);
+    }
+
+    match (mode, key, modifiers) {
+        // Mode transitions
+        (AppMode::ReadOnly, KeyCode::Char('i'), _) => { editor.enter_edit_mode(); editor.clear_command_buffer(); }
+        (AppMode::ReadOnly, KeyCode::Char('a'), _) => { editor.enter_append_mode(); editor.clear_command_buffer(); }
+        (AppMode::ReadOnly, KeyCode::Char('A'), _) => { editor.move_line_end(); editor.enter_edit_mode(); editor.clear_command_buffer(); }
+        (_, KeyCode::Esc, _) => { if mode == AppMode::Edit { editor.exit_edit_mode(); } else { editor.clear_command_buffer(); } }
+
+        // Validation commands
+        (AppMode::ReadOnly, KeyCode::F(1), _) => { editor.toggle_validation(); }
+
+        // Movement in ReadOnly mode
+        (AppMode::ReadOnly, KeyCode::Char('h'), _) | (AppMode::ReadOnly, KeyCode::Left, _) => { editor.move_left(); editor.clear_command_buffer(); }
+        (AppMode::ReadOnly, KeyCode::Char('l'), _) | (AppMode::ReadOnly, KeyCode::Right, _) => { editor.move_right(); editor.clear_command_buffer(); }
+        (AppMode::ReadOnly, KeyCode::Char('j'), _) | (AppMode::ReadOnly, KeyCode::Down, _) => { editor.move_down(); editor.clear_command_buffer(); }
+        (AppMode::ReadOnly, KeyCode::Char('k'), _) | (AppMode::ReadOnly, KeyCode::Up, _) => { editor.move_up(); editor.clear_command_buffer(); }
+
+        // Movement in Edit mode
+        (AppMode::Edit, KeyCode::Left, _) => { editor.move_left(); }
+        (AppMode::Edit, KeyCode::Right, _) => { editor.move_right(); }
+        (AppMode::Edit, KeyCode::Up, _) => { editor.move_up(); }
+        (AppMode::Edit, KeyCode::Down, _) => { editor.move_down(); }
+
+        // Delete operations
+        (AppMode::Edit, KeyCode::Backspace, _) => { editor.delete_backward()?; }
+        (AppMode::Edit, KeyCode::Delete, _) => { editor.delete_forward()?; }
+
+        // Tab navigation
+        (_, KeyCode::Tab, _) => { editor.next_field(); }
+        (_, KeyCode::BackTab, _) => { editor.prev_field(); }
+
+        // Character input
+        (AppMode::Edit, KeyCode::Char(c), m) if !m.contains(KeyModifiers::CONTROL) => {
+            editor.insert_char(c)?;
+        }
+
+        // Debug info
+        (AppMode::ReadOnly, KeyCode::Char('?'), _) => {
+            let summary = editor.editor.validation_summary();
+            editor.set_debug_message(format!(
+                "Field {}/{}, Pos {}, Mode: {:?}, Advanced patterns: {} configured",
+                editor.current_field() + 1,
+                editor.data_provider().field_count(),
+                editor.cursor_position(),
+                editor.mode(),
+                summary.total_fields
+            ));
+        }
+
+        _ => {
+            if editor.has_pending_command() {
+                editor.clear_command_buffer();
+                editor.set_debug_message("Invalid command sequence".to_string());
+            }
+        }
+    }
+
+    Ok(true)
+}
+
+fn run_app<B: Backend>(
+    terminal: &mut Terminal<B>,
+    mut editor: AdvancedPatternFormEditor<AdvancedPatternData>,
+) -> io::Result<()> {
+    loop {
+        terminal.draw(|f| ui(f, &editor))?;
+
+        if let Event::Key(key) = event::read()? {
+            match handle_key_press(key.code, key.modifiers, &mut editor) {
+                Ok(should_continue) => {
+                    if !should_continue {
+                        break;
+                    }
+                }
+                Err(e) => {
+                    editor.set_debug_message(format!("Error: {}", e));
+                }
+            }
+        }
+    }
+
+    Ok(())
+}
+
+fn ui(f: &mut Frame, editor: &AdvancedPatternFormEditor<AdvancedPatternData>) {
+    let chunks = Layout::default()
+        .direction(Direction::Vertical)
+        .constraints([Constraint::Min(8), Constraint::Length(15)])
+        .split(f.area());
+
+    render_canvas_default(f, chunks[0], &editor.editor);
+    render_advanced_validation_status(f, chunks[1], editor);
+}
+
+fn render_advanced_validation_status(
+    f: &mut Frame,
+    area: Rect,
+    editor: &AdvancedPatternFormEditor<AdvancedPatternData>,
+) {
+    let chunks = Layout::default()
+        .direction(Direction::Vertical)
+        .constraints([
+            Constraint::Length(3), // Status bar
+            Constraint::Length(5), // Validation summary
+            Constraint::Length(7), // Help
+        ])
+        .split(area);
+
+    // Status bar
+    let mode_text = match editor.mode() {
+        AppMode::Edit => "INSERT",
+        AppMode::ReadOnly => "NORMAL",
+        _ => "OTHER",
+    };
+
+    let validation_status = editor.get_validation_status();
+    let status_text = format!("-- {} -- {} | Advanced Patterns: {}", mode_text, editor.debug_message(), validation_status);
+
+    let status = Paragraph::new(Line::from(Span::raw(status_text)))
+        .block(Block::default().borders(Borders::ALL).title("🚀 Advanced Pattern Validation"));
+
+    f.render_widget(status, chunks[0]);
+
+    // Enhanced validation summary
+    let summary = editor.editor.validation_summary();
+    let field_info = match editor.current_field() {
+        0 => "Time format (HH:MM) - Tests exact chars + numeric ranges",
+        1 => "Hex color (#RRGGBB) - Tests OneOf filter with case insensitive",
+        2 => "IPv4 address - Tests complex dot positioning",
+        3 => "Product code (ABC-123-XYZ) - Tests section-based patterns",
+        4 => "Date code (2024W15) - Tests From position filtering",
+        5 => "Binary input - Tests limited character set (0,1 only)",
+        6 => "Complex ID - Tests overlapping/conflicting rules",
+        7 => "Custom pattern - Tests advanced custom validation logic",
+        _ => "Unknown field",
+    };
+
+    let summary_text = if editor.validation_enabled {
+        format!(
+            "📊 Advanced Pattern Summary: {} fields with complex rules\n\
+             Current Field: {}\n\
+             ✅ Valid: {}  ⚠️  Warnings: {}  ❌ Errors: {}  📈 Progress: {:.0}%\n\
+             🎯 Pattern Focus: {}",
+            summary.total_fields,
+            editor.current_field() + 1,
+            summary.valid_fields,
+            summary.warning_fields,
+            summary.error_fields,
+            summary.completion_percentage() * 100.0,
+            field_info
+        )
+    } else {
+        "❌ Advanced pattern validation is DISABLED\nPress F1 to enable and see the magic!".to_string()
+    };
+
+    let summary_style = if summary.has_errors() {
+        Style::default().fg(Color::Red)
+    } else if summary.has_warnings() {
+        Style::default().fg(Color::Yellow)
+    } else {
+        Style::default().fg(Color::Green)
+    };
+
+    let validation_summary = Paragraph::new(summary_text)
+        .block(Block::default().borders(Borders::ALL).title("🎯 Advanced Pattern Analysis"))
+        .style(summary_style)
+        .wrap(Wrap { trim: true });
+
+    f.render_widget(validation_summary, chunks[1]);
+
+    // Enhanced help text
+    let help_text = match editor.mode() {
+        AppMode::ReadOnly => {
+            "🚀 ADVANCED PATTERN SHOWCASE - Each field demonstrates different edge cases!\n\
+             🕐 Time: Numeric+exact chars  🎨 Hex: OneOf with case-insensitive  🌐 IPv4: Complex positioning\n\
+             🏷️  Product: Multi-section rules  📅 Date: From-position filtering  🔢 Binary: Limited charset\n\
+             🎯 Complex: Overlapping rules  🚀 Custom: Advanced logic functions\n\
+             \n\
+             Movement: hjkl/arrows=move, Tab/Shift+Tab=fields, i/a=insert, F1=toggle, ?=info"
+        }
+        AppMode::Edit => {
+            "✏️  INSERT MODE - Testing advanced pattern validation!\n\
+             Each character is validated against complex rules in real-time\n\
+             Try entering invalid characters to see detailed error messages\n\
+             arrows=move, Backspace/Del=delete, Esc=normal, Tab=next field"
+        }
+        _ => "🚀 Advanced Pattern Validation Active!"
+    };
+
+    let help = Paragraph::new(help_text)
+        .block(Block::default().borders(Borders::ALL).title("🎯 Advanced Pattern Commands & Info"))
+        .style(Style::default().fg(Color::Gray))
+        .wrap(Wrap { trim: true });
+
+    f.render_widget(help, chunks[2]);
+}
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    println!("🚀 Canvas Advanced Pattern Validation Demo");
+    println!("✅ validation feature: ENABLED");
+    println!("✅ gui feature: ENABLED");
+    println!("🎯 Advanced pattern filtering: ACTIVE");
+    println!("🧪 Edge cases and complex patterns: READY");
+    println!("💡 Each field showcases different validation capabilities!");
+    println!();
+
+    enable_raw_mode()?;
+    let mut stdout = io::stdout();
+    execute!(stdout, EnterAlternateScreen, EnableMouseCapture)?;
+    let backend = CrosstermBackend::new(stdout);
+    let mut terminal = Terminal::new(backend)?;
+
+    let data = AdvancedPatternData::new();
+    let editor = AdvancedPatternFormEditor::new(data);
+
+    let res = run_app(&mut terminal, editor);
+
+    disable_raw_mode()?;
+    execute!(
+        terminal.backend_mut(),
+        LeaveAlternateScreen,
+        DisableMouseCapture
+    )?;
+    terminal.show_cursor()?;
+
+    if let Err(err) = res {
+        println!("{:?}", err);
+    }
+
+    println!("🚀 Advanced pattern validation demo completed!");
+    println!("🎯 Hope you enjoyed seeing all the edge cases in action!");
+    Ok(())
+}

canvas/examples/validation_3.rs

@@ -0,0 +1,712 @@
+// examples/validation_3.rs
+//! Comprehensive Display Mask Features Demo
+//!
+//! This example showcases the full power of the display mask system (Feature 3)
+//! demonstrating visual formatting that keeps business logic clean.
+//!
+//! Key Features Demonstrated:
+//! - Dynamic vs Template display modes
+//! - Custom patterns for different data types
+//! - Custom input characters and separators
+//! - Custom placeholder characters
+//! - Real-time visual formatting with clean raw data
+//! - Cursor movement through formatted displays
+//! - 🔥 CRITICAL: Perfect mask/character-limit coordination to prevent invisible character bugs
+//!
+//! ⚠️  IMPORTANT BUG PREVENTION:
+//! This example demonstrates the CORRECT way to configure masks with character limits.
+//! Each mask's input position count EXACTLY matches its character limit to prevent
+//! the critical bug where users can type more characters than they can see.
+//!
+//! Run with: cargo run --example validation_3 --features "gui,validation"
+
+// REQUIRE validation and gui features for mask functionality
+#[cfg(not(all(feature = "validation", feature = "gui")))]
+compile_error!(
+    "This example requires the 'validation' and 'gui' features. \
+     Run with: cargo run --example validation_3 --features \"gui,validation\""
+);
+
+use std::io;
+use crossterm::{
+    event::{
+        self, DisableMouseCapture, EnableMouseCapture, Event, KeyCode, KeyModifiers,
+    },
+    execute,
+    terminal::{
+        disable_raw_mode, enable_raw_mode, EnterAlternateScreen, LeaveAlternateScreen,
+    },
+};
+use ratatui::{
+    backend::{Backend, CrosstermBackend},
+    layout::{Constraint, Direction, Layout, Rect},
+    style::{Color, Style},
+    text::{Line, Span},
+    widgets::{Block, Borders, Paragraph, Wrap},
+    Frame, Terminal,
+};
+
+use canvas::{
+    canvas::{
+        gui::render_canvas_default,
+        modes::AppMode,
+    },
+    DataProvider, FormEditor,
+    ValidationConfig, ValidationConfigBuilder, DisplayMask,
+    validation::mask::MaskDisplayMode,
+};
+
+// Enhanced FormEditor wrapper for mask demonstration
+struct MaskDemoFormEditor<D: DataProvider> {
+    editor: FormEditor<D>,
+    debug_message: String,
+    command_buffer: String,
+    validation_enabled: bool,
+    show_raw_data: bool,
+}
+
+impl<D: DataProvider> MaskDemoFormEditor<D> {
+    fn new(data_provider: D) -> Self {
+        let mut editor = FormEditor::new(data_provider);
+        editor.set_validation_enabled(true);
+
+        Self {
+            editor,
+            debug_message: "🎭 Display Mask Demo - Visual formatting with clean business logic!".to_string(),
+            command_buffer: String::new(),
+            validation_enabled: true,
+            show_raw_data: false,
+        }
+    }
+
+    // === COMMAND BUFFER HANDLING ===
+    fn clear_command_buffer(&mut self) { self.command_buffer.clear(); }
+    fn add_to_command_buffer(&mut self, ch: char) { self.command_buffer.push(ch); }
+    fn get_command_buffer(&self) -> &str { &self.command_buffer }
+    fn has_pending_command(&self) -> bool { !self.command_buffer.is_empty() }
+
+    // === MASK CONTROL ===
+    fn toggle_validation(&mut self) {
+        self.validation_enabled = !self.validation_enabled;
+        self.editor.set_validation_enabled(self.validation_enabled);
+        if self.validation_enabled {
+            self.debug_message = "✅ Display Masks ENABLED - See visual formatting in action!".to_string();
+        } else {
+            self.debug_message = "❌ Display Masks DISABLED - Raw text only".to_string();
+        }
+    }
+
+    fn toggle_raw_data_view(&mut self) {
+        self.show_raw_data = !self.show_raw_data;
+        if self.show_raw_data {
+            self.debug_message = "👁️ Showing RAW business data (what's actually stored)".to_string();
+        } else {
+            self.debug_message = "🎭 Showing FORMATTED display (what users see)".to_string();
+        }
+    }
+
+    fn get_current_field_info(&self) -> (String, String, String) {
+        let field_index = self.editor.current_field();
+        let raw_data = self.editor.current_text();
+        let display_data = if self.validation_enabled {
+            self.editor.current_display_text()
+        } else {
+            raw_data.to_string()
+        };
+
+        let mask_info = if let Some(config) = self.editor.validation_state().get_field_config(field_index) {
+            if let Some(mask) = &config.display_mask {
+                format!("Pattern: '{}', Mode: {:?}", 
+                       mask.pattern(), 
+                       mask.display_mode())
+            } else {
+                "No mask configured".to_string()
+            }
+        } else {
+            "No validation config".to_string()
+        };
+
+        (raw_data.to_string(), display_data, mask_info)
+    }
+
+    // === ENHANCED MOVEMENT WITH MASK AWARENESS ===
+    fn move_left(&mut self) { 
+        self.editor.move_left(); 
+        self.update_cursor_info();
+    }
+
+    fn move_right(&mut self) { 
+        self.editor.move_right(); 
+        self.update_cursor_info();
+    }
+
+    fn move_up(&mut self) {
+        match self.editor.move_up() {
+            Ok(()) => { self.update_field_info(); }
+            Err(e) => { self.debug_message = format!("🚫 Field switch blocked: {}", e); }
+        }
+    }
+
+    fn move_down(&mut self) {
+        match self.editor.move_down() {
+            Ok(()) => { self.update_field_info(); }
+            Err(e) => { self.debug_message = format!("🚫 Field switch blocked: {}", e); }
+        }
+    }
+
+    fn move_line_start(&mut self) { 
+        self.editor.move_line_start(); 
+        self.update_cursor_info();
+    }
+
+    fn move_line_end(&mut self) { 
+        self.editor.move_line_end(); 
+        self.update_cursor_info();
+    }
+
+    fn update_cursor_info(&mut self) {
+        if self.validation_enabled {
+            let raw_pos = self.editor.cursor_position();
+            let display_pos = self.editor.display_cursor_position();
+            if raw_pos != display_pos {
+                self.debug_message = format!("📍 Cursor: Raw pos {} → Display pos {} (mask active)", raw_pos, display_pos);
+            } else {
+                self.debug_message = format!("📍 Cursor at position {} (no mask offset)", raw_pos);
+            }
+        }
+    }
+
+    fn update_field_info(&mut self) {
+        let field_name = self.editor.data_provider().field_name(self.editor.current_field());
+        self.debug_message = format!("📝 Switched to: {}", field_name);
+    }
+
+    // === MODE TRANSITIONS ===
+    fn enter_edit_mode(&mut self) {
+        self.editor.enter_edit_mode();
+        self.debug_message = "✏️ INSERT MODE - Type to see mask formatting in real-time".to_string();
+    }
+
+    fn enter_append_mode(&mut self) {
+        self.editor.enter_append_mode();
+        self.debug_message = "✏️ INSERT (append) - Mask formatting active".to_string();
+    }
+
+    fn exit_edit_mode(&mut self) {
+        self.editor.exit_edit_mode();
+        self.debug_message = "🔒 NORMAL MODE - Press 'r' to see raw data, 'm' for mask info".to_string();
+    }
+
+    fn insert_char(&mut self, ch: char) -> anyhow::Result<()> {
+        let result = self.editor.insert_char(ch);
+        if result.is_ok() {
+            let (raw, display, _) = self.get_current_field_info();
+            if raw != display {
+                self.debug_message = format!("✏️ Added '{}': Raw='{}' Display='{}'", ch, raw, display);
+            } else {
+                self.debug_message = format!("✏️ Added '{}': '{}'", ch, raw);
+            }
+        }
+        Ok(result?)
+    }
+
+    // === DELETE OPERATIONS ===
+    fn delete_backward(&mut self) -> anyhow::Result<()> {
+        let result = self.editor.delete_backward();
+        if result.is_ok() {
+            self.debug_message = "⌫ Character deleted".to_string();
+            self.update_cursor_info();
+        }
+        Ok(result?)
+    }
+
+    fn delete_forward(&mut self) -> anyhow::Result<()> {
+        let result = self.editor.delete_forward();
+        if result.is_ok() {
+            self.debug_message = "⌦ Character deleted".to_string();
+            self.update_cursor_info();
+        }
+        Ok(result?)
+    }
+
+    // === DELEGATE TO ORIGINAL EDITOR ===
+    fn current_field(&self) -> usize { self.editor.current_field() }
+    fn cursor_position(&self) -> usize { self.editor.cursor_position() }
+    fn mode(&self) -> AppMode { self.editor.mode() }
+    fn current_text(&self) -> &str { self.editor.current_text() }
+    fn data_provider(&self) -> &D { self.editor.data_provider() }
+    fn ui_state(&self) -> &canvas::EditorState { self.editor.ui_state() }
+    fn set_mode(&mut self, mode: AppMode) { self.editor.set_mode(mode); }
+
+    fn next_field(&mut self) {
+        match self.editor.next_field() {
+            Ok(()) => { self.update_field_info(); }
+            Err(e) => { self.debug_message = format!("🚫 Cannot move to next field: {}", e); }
+        }
+    }
+
+    fn prev_field(&mut self) {
+        match self.editor.prev_field() {
+            Ok(()) => { self.update_field_info(); }
+            Err(e) => { self.debug_message = format!("🚫 Cannot move to previous field: {}", e); }
+        }
+    }
+
+    // === STATUS AND DEBUG ===
+    fn set_debug_message(&mut self, msg: String) { self.debug_message = msg; }
+    fn debug_message(&self) -> &str { &self.debug_message }
+
+    fn show_mask_details(&mut self) {
+        let (raw, display, mask_info) = self.get_current_field_info();
+        self.debug_message = format!("🔍 Field {}: {} | Raw: '{}' Display: '{}'", 
+                                   self.current_field() + 1, mask_info, raw, display);
+    }
+
+    fn get_mask_status(&self) -> String {
+        if !self.validation_enabled {
+            return "❌ DISABLED".to_string();
+        }
+
+        let field_count = self.editor.data_provider().field_count();
+        let mut mask_count = 0;
+        for i in 0..field_count {
+            if let Some(config) = self.editor.validation_state().get_field_config(i) {
+                if config.display_mask.is_some() {
+                    mask_count += 1;
+                }
+            }
+        }
+
+        format!("🎭 {} MASKS", mask_count)
+    }
+}
+
+// Demo data with comprehensive mask examples
+struct MaskDemoData {
+    fields: Vec<(String, String)>,
+}
+
+impl MaskDemoData {
+    fn new() -> Self {
+        Self {
+            fields: vec![
+                ("📞 Phone (Dynamic)".to_string(), "".to_string()),
+                ("📞 Phone (Template)".to_string(), "".to_string()),
+                ("📅 Date US (MM/DD/YYYY)".to_string(), "".to_string()),
+                ("📅 Date EU (DD.MM.YYYY)".to_string(), "".to_string()),
+                ("📅 Date ISO (YYYY-MM-DD)".to_string(), "".to_string()),
+                ("🏛️ SSN (XXX-XX-XXXX)".to_string(), "".to_string()),
+                ("💳 Credit Card".to_string(), "".to_string()),
+                ("🏢 Employee ID (EMP-####)".to_string(), "".to_string()),
+                ("📦 Product Code (ABC###XYZ)".to_string(), "".to_string()),
+                ("🌈 Custom Separators".to_string(), "".to_string()),
+                ("⭐ Custom Placeholders".to_string(), "".to_string()),
+                ("🎯 Mixed Input Chars".to_string(), "".to_string()),
+            ],
+        }
+    }
+}
+
+impl DataProvider for MaskDemoData {
+    fn field_count(&self) -> usize { self.fields.len() }
+    fn field_name(&self, index: usize) -> &str { &self.fields[index].0 }
+    fn field_value(&self, index: usize) -> &str { &self.fields[index].1 }
+    fn set_field_value(&mut self, index: usize, value: String) { self.fields[index].1 = value; }
+
+    fn validation_config(&self, field_index: usize) -> Option<ValidationConfig> {
+        match field_index {
+            0 => {
+                // 📞 Phone (Dynamic) - FIXED: Perfect mask/limit coordination
+                let phone_mask = DisplayMask::new("(###) ###-####", '#');
+                Some(ValidationConfigBuilder::new()
+                    .with_display_mask(phone_mask)
+                    .with_max_length(10)  // ✅ CRITICAL: Exactly matches 10 input positions
+                    .build())
+            }
+            1 => {
+                // 📞 Phone (Template) - FIXED: Perfect mask/limit coordination  
+                let phone_template = DisplayMask::new("(###) ###-####", '#')
+                    .with_template('_');
+                Some(ValidationConfigBuilder::new()
+                    .with_display_mask(phone_template)  
+                    .with_max_length(10)  // ✅ CRITICAL: Exactly matches 10 input positions
+                    .build())
+            }
+            2 => {
+                // 📅 Date US (MM/DD/YYYY) - American date format
+                let us_date = DisplayMask::new("##/##/####", '#');
+                Some(ValidationConfig::with_mask(us_date))
+            }
+            3 => {
+                // 📅 Date EU (DD.MM.YYYY) - European date format with dots
+                let eu_date = DisplayMask::new("##.##.####", '#')
+                    .with_template('•');
+                Some(ValidationConfig::with_mask(eu_date))
+            }
+            4 => {
+                // 📅 Date ISO (YYYY-MM-DD) - ISO date format
+                let iso_date = DisplayMask::new("####-##-##", '#')
+                    .with_template('-');
+                Some(ValidationConfig::with_mask(iso_date))
+            }
+            5 => {
+                // 🏛️ SSN using custom input character 'X' - FIXED: Perfect coordination
+                let ssn_mask = DisplayMask::new("XXX-XX-XXXX", 'X');
+                Some(ValidationConfigBuilder::new()
+                    .with_display_mask(ssn_mask)
+                    .with_max_length(9)  // ✅ CRITICAL: Exactly matches 9 input positions  
+                    .build())
+            }
+            6 => {
+                // 💳 Credit Card (16 digits with spaces) - FIXED: Perfect coordination
+                let cc_mask = DisplayMask::new("#### #### #### ####", '#')
+                    .with_template('•');
+                Some(ValidationConfigBuilder::new()
+                    .with_display_mask(cc_mask)
+                    .with_max_length(16)  // ✅ CRITICAL: Exactly matches 16 input positions
+                    .build())
+            }
+            7 => {
+                // 🏢 Employee ID with business prefix
+                let emp_id = DisplayMask::new("EMP-####", '#');
+                Some(ValidationConfig::with_mask(emp_id))
+            }
+            8 => {
+                // 📦 Product Code with mixed letters and numbers
+                let product_code = DisplayMask::new("ABC###XYZ", '#');
+                Some(ValidationConfig::with_mask(product_code))
+            }
+            9 => {
+                // 🌈 Custom Separators - Using | and ~ as separators
+                let custom_sep = DisplayMask::new("##|##~####", '#')
+                    .with_template('?');
+                Some(ValidationConfig::with_mask(custom_sep))
+            }
+            10 => {
+                // ⭐ Custom Placeholders - Using different placeholder characters
+                let custom_placeholder = DisplayMask::new("##-##-##", '#')
+                    .with_template('★');
+                Some(ValidationConfig::with_mask(custom_placeholder))
+            }
+            11 => {
+                // 🎯 Mixed Input Characters - Using 'N' for numbers
+                let mixed_input = DisplayMask::new("ID:NNN-NNN", 'N');
+                Some(ValidationConfig::with_mask(mixed_input))
+            }
+            _ => None,
+        }
+    }
+}
+
+// Enhanced key handling with mask-specific commands
+fn handle_key_press(
+    key: KeyCode,
+    modifiers: KeyModifiers,
+    editor: &mut MaskDemoFormEditor<MaskDemoData>,
+) -> anyhow::Result<bool> {
+    let mode = editor.mode();
+
+    // Quit handling
+    if (key == KeyCode::Char('q') && modifiers.contains(KeyModifiers::CONTROL))
+        || (key == KeyCode::Char('c') && modifiers.contains(KeyModifiers::CONTROL))
+        || key == KeyCode::F(10)
+    {
+        return Ok(false);
+    }
+
+    match (mode, key, modifiers) {
+        // === MODE TRANSITIONS ===
+        (AppMode::ReadOnly, KeyCode::Char('i'), _) => {
+            editor.enter_edit_mode();
+            editor.clear_command_buffer();
+        }
+        (AppMode::ReadOnly, KeyCode::Char('a'), _) => {
+            editor.enter_append_mode();
+            editor.clear_command_buffer();
+        }
+        (AppMode::ReadOnly, KeyCode::Char('A'), _) => {
+            editor.move_line_end();
+            editor.enter_edit_mode();
+            editor.clear_command_buffer();
+        }
+
+        // Escape: Exit edit mode
+        (_, KeyCode::Esc, _) => {
+            if mode == AppMode::Edit {
+                editor.exit_edit_mode();
+            } else {
+                editor.clear_command_buffer();
+            }
+        }
+
+        // === MASK SPECIFIC COMMANDS ===
+        (AppMode::ReadOnly, KeyCode::Char('m'), _) => {
+            editor.show_mask_details();
+            editor.clear_command_buffer();
+        }
+        (AppMode::ReadOnly, KeyCode::Char('r'), _) => {
+            editor.toggle_raw_data_view();
+            editor.clear_command_buffer();
+        }
+        (AppMode::ReadOnly, KeyCode::F(1), _) => {
+            editor.toggle_validation();
+        }
+
+        // === MOVEMENT ===
+        (AppMode::ReadOnly, KeyCode::Char('h'), _) | (AppMode::ReadOnly, KeyCode::Left, _) => {
+            editor.move_left();
+            editor.clear_command_buffer();
+        }
+        (AppMode::ReadOnly, KeyCode::Char('l'), _) | (AppMode::ReadOnly, KeyCode::Right, _) => {
+            editor.move_right();
+            editor.clear_command_buffer();
+        }
+        (AppMode::ReadOnly, KeyCode::Char('j'), _) | (AppMode::ReadOnly, KeyCode::Down, _) => {
+            editor.move_down();
+            editor.clear_command_buffer();
+        }
+        (AppMode::ReadOnly, KeyCode::Char('k'), _) | (AppMode::ReadOnly, KeyCode::Up, _) => {
+            editor.move_up();
+            editor.clear_command_buffer();
+        }
+
+        // Line movement
+        (AppMode::ReadOnly, KeyCode::Char('0'), _) => {
+            editor.move_line_start();
+            editor.clear_command_buffer();
+        }
+        (AppMode::ReadOnly, KeyCode::Char('$'), _) => {
+            editor.move_line_end();
+            editor.clear_command_buffer();
+        }
+
+        // === EDIT MODE MOVEMENT ===
+        (AppMode::Edit, KeyCode::Left, _) => { editor.move_left(); }
+        (AppMode::Edit, KeyCode::Right, _) => { editor.move_right(); }
+        (AppMode::Edit, KeyCode::Up, _) => { editor.move_up(); }
+        (AppMode::Edit, KeyCode::Down, _) => { editor.move_down(); }
+
+        // === DELETE OPERATIONS ===
+        (AppMode::Edit, KeyCode::Backspace, _) => { editor.delete_backward()?; }
+        (AppMode::Edit, KeyCode::Delete, _) => { editor.delete_forward()?; }
+
+        // === TAB NAVIGATION ===
+        (_, KeyCode::Tab, _) => { editor.next_field(); }
+        (_, KeyCode::BackTab, _) => { editor.prev_field(); }
+
+        // === CHARACTER INPUT ===
+        (AppMode::Edit, KeyCode::Char(c), m) if !m.contains(KeyModifiers::CONTROL) => {
+            editor.insert_char(c)?;
+        }
+
+        // === DEBUG/INFO COMMANDS ===
+        (AppMode::ReadOnly, KeyCode::Char('?'), _) => {
+            let (raw, display, mask_info) = editor.get_current_field_info();
+            editor.set_debug_message(format!(
+                "Field {}/{}, Cursor {}, {}, Raw: '{}', Display: '{}'",
+                editor.current_field() + 1,
+                editor.data_provider().field_count(),
+                editor.cursor_position(),
+                mask_info,
+                raw,
+                display
+            ));
+        }
+
+        _ => {
+            if editor.has_pending_command() {
+                editor.clear_command_buffer();
+                editor.set_debug_message("Invalid command sequence".to_string());
+            }
+        }
+    }
+
+    Ok(true)
+}
+
+fn run_app<B: Backend>(
+    terminal: &mut Terminal<B>,
+    mut editor: MaskDemoFormEditor<MaskDemoData>,
+) -> io::Result<()> {
+    loop {
+        terminal.draw(|f| ui(f, &editor))?;
+
+        if let Event::Key(key) = event::read()? {
+            match handle_key_press(key.code, key.modifiers, &mut editor) {
+                Ok(should_continue) => {
+                    if !should_continue {
+                        break;
+                    }
+                }
+                Err(e) => {
+                    editor.set_debug_message(format!("Error: {}", e));
+                }
+            }
+        }
+    }
+
+    Ok(())
+}
+
+fn ui(f: &mut Frame, editor: &MaskDemoFormEditor<MaskDemoData>) {
+    let chunks = Layout::default()
+        .direction(Direction::Vertical)
+        .constraints([Constraint::Min(8), Constraint::Length(16)])
+        .split(f.area());
+
+    render_enhanced_canvas(f, chunks[0], editor);
+    render_mask_status(f, chunks[1], editor);
+}
+
+fn render_enhanced_canvas(
+    f: &mut Frame,
+    area: Rect,
+    editor: &MaskDemoFormEditor<MaskDemoData>,
+) {
+    render_canvas_default(f, area, &editor.editor);
+}
+
+fn render_mask_status(
+    f: &mut Frame,
+    area: Rect,
+    editor: &MaskDemoFormEditor<MaskDemoData>,
+) {
+    let chunks = Layout::default()
+        .direction(Direction::Vertical)
+        .constraints([
+            Constraint::Length(3), // Status bar
+            Constraint::Length(6), // Data comparison
+            Constraint::Length(7), // Help
+        ])
+        .split(area);
+
+    // Status bar with mask information
+    let mode_text = match editor.mode() {
+        AppMode::Edit => "INSERT",
+        AppMode::ReadOnly => "NORMAL",
+        _ => "OTHER",
+    };
+
+    let mask_status = editor.get_mask_status();
+    let status_text = format!("-- {} -- {} | Masks: {} | View: {}", 
+                             mode_text, 
+                             editor.debug_message(), 
+                             mask_status,
+                             if editor.show_raw_data { "RAW" } else { "FORMATTED" });
+
+    let status = Paragraph::new(Line::from(Span::raw(status_text)))
+        .block(Block::default().borders(Borders::ALL).title("🎭 Display Mask Demo"));
+
+    f.render_widget(status, chunks[0]);
+
+    // Data comparison showing raw vs display
+    let (raw_data, display_data, mask_info) = editor.get_current_field_info();
+    let field_name = editor.data_provider().field_name(editor.current_field());
+    
+    let comparison_text = format!(
+        "📝 Current Field: {}\n\
+         🔧 Mask Config: {}\n\
+         \n\
+         💾 Raw Business Data: '{}' ← What's actually stored in your database\n\
+         🎭 Formatted Display: '{}' ← What users see in the interface\n\
+         📍 Cursor: Raw pos {} → Display pos {}",
+        field_name,
+        mask_info,
+        raw_data,
+        display_data,
+        editor.cursor_position(),
+        editor.editor.display_cursor_position()
+    );
+
+    let comparison_style = if raw_data != display_data {
+        Style::default().fg(Color::Green) // Green when mask is active
+    } else {
+        Style::default().fg(Color::Gray) // Gray when no formatting
+    };
+
+    let data_comparison = Paragraph::new(comparison_text)
+        .block(Block::default().borders(Borders::ALL).title("📊 Raw Data vs Display Formatting"))
+        .style(comparison_style)
+        .wrap(Wrap { trim: true });
+
+    f.render_widget(data_comparison, chunks[1]);
+
+    // Enhanced help text
+    let help_text = match editor.mode() {
+        AppMode::ReadOnly => {
+            "🎭 MASK DEMO: See how visual formatting keeps business logic clean!\n\
+             \n\
+             📱 Try different fields to see various mask patterns:\n\
+             • Dynamic vs Template modes  • Custom separators  • Different input chars\n\
+             \n\
+             Commands: i/a=insert, m=mask details, r=toggle raw/display view\n\
+             Movement: hjkl/arrows=move, 0=$=line start/end, Tab=next field, F1=toggle masks\n\
+             ?=detailed info, Ctrl+C=quit"
+        }
+        AppMode::Edit => {
+            "✏️ INSERT MODE - Type to see real-time mask formatting!\n\
+             \n\
+             🔥 Key Features in Action:\n\
+             • Separators auto-appear as you type  • Cursor skips over separators\n\
+             • Template fields show placeholders  • Raw data stays clean for business logic\n\
+             \n\
+             arrows=move through mask, Backspace/Del=delete, Esc=normal, Tab=next field\n\
+             Notice how cursor position maps between raw data and display!"
+        }
+        _ => "🎭 Display Mask Demo Active!"
+    };
+
+    let help = Paragraph::new(help_text)
+        .block(Block::default().borders(Borders::ALL).title("🚀 Mask Features & Commands"))
+        .style(Style::default().fg(Color::Gray))
+        .wrap(Wrap { trim: true });
+
+    f.render_widget(help, chunks[2]);
+}
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    // Print feature status
+    println!("🎭 Canvas Display Mask Demo (Feature 3)");
+    println!("✅ validation feature: ENABLED");
+    println!("✅ gui feature: ENABLED");
+    println!("🎭 Display masks: ACTIVE");
+    println!("🔥 Key Benefits Demonstrated:");
+    println!("   • Clean separation: Visual formatting ≠ Business logic");
+    println!("   • User-friendly: Pretty displays with automatic cursor handling");
+    println!("   • Flexible: Custom patterns, separators, and placeholders");
+    println!("   • Transparent: Library handles all complexity, API stays simple");
+    println!();
+    println!("💡 Try typing in different fields to see mask magic!");
+    println!("   📞 Phone fields show dynamic vs template modes");
+    println!("   📅 Date fields show different regional formats");
+    println!("   💳 Credit card shows spaced formatting");
+    println!("   ⭐ Custom fields show advanced separator/placeholder options");
+    println!();
+
+    enable_raw_mode()?;
+    let mut stdout = io::stdout();
+    execute!(stdout, EnterAlternateScreen, EnableMouseCapture)?;
+    let backend = CrosstermBackend::new(stdout);
+    let mut terminal = Terminal::new(backend)?;
+
+    let data = MaskDemoData::new();
+    let editor = MaskDemoFormEditor::new(data);
+
+    let res = run_app(&mut terminal, editor);
+
+    disable_raw_mode()?;
+    execute!(
+        terminal.backend_mut(),
+        LeaveAlternateScreen,
+        DisableMouseCapture
+    )?;
+    terminal.show_cursor()?;
+
+    if let Err(err) = res {
+        println!("{:?}", err);
+    }
+
+    println!("🎭 Display mask demo completed!");
+    println!("🏆 You've seen how masks provide beautiful UX while keeping business logic clean!");
+    Ok(())
+}

canvas/examples/validation_4.rs

@@ -0,0 +1,738 @@
+/* examples/validation_4.rs
+   Enhanced Feature 4 Demo: Multiple custom formatters with comprehensive edge cases
+   
+   Demonstrates:
+   - Multiple formatter types: PSC, Phone, Credit Card, Date
+   - Edge case handling: incomplete input, invalid chars, overflow
+   - Real-time validation feedback and format preview
+   - Advanced cursor position mapping
+   - Raw vs formatted data separation
+   - Error handling and fallback behavior
+
+*/
+
+#![allow(clippy::needless_return)]
+
+#[cfg(not(all(feature = "validation", feature = "gui")))]
+compile_error!(
+    "This example requires the 'validation' and 'gui' features. \
+     Run with: cargo run --example validation_4 --features \"gui,validation\""
+);
+
+use std::io;
+use std::sync::Arc;
+
+use crossterm::{
+    event::{
+        self, DisableMouseCapture, EnableMouseCapture, Event, KeyCode, KeyModifiers,
+    },
+    execute,
+    terminal::{disable_raw_mode, enable_raw_mode, EnterAlternateScreen, LeaveAlternateScreen},
+};
+use ratatui::{
+    backend::{Backend, CrosstermBackend},
+    layout::{Constraint, Direction, Layout, Rect},
+    style::{Color, Style},
+    text::{Line, Span},
+    widgets::{Block, Borders, Paragraph, Wrap},
+    Frame, Terminal,
+};
+
+use canvas::{
+    canvas::{gui::render_canvas_default, modes::AppMode},
+    DataProvider, FormEditor,
+    ValidationConfig, ValidationConfigBuilder,
+    CustomFormatter, FormattingResult,
+};
+
+/// PSC (Postal Code) Formatter: "01001" -> "010 01"
+struct PSCFormatter;
+
+impl CustomFormatter for PSCFormatter {
+    fn format(&self, raw: &str) -> FormattingResult {
+        if raw.is_empty() {
+            return FormattingResult::success("");
+        }
+        
+        // Validate: only digits allowed
+        if !raw.chars().all(|c| c.is_ascii_digit()) {
+            return FormattingResult::error("PSC must contain only digits");
+        }
+        
+        let len = raw.chars().count();
+        match len {
+            0 => FormattingResult::success(""),
+            1..=3 => FormattingResult::success(raw),
+            4 => FormattingResult::warning(
+                format!("{} ", &raw[..3]), 
+                "PSC incomplete (4/5 digits)"
+            ),
+            5 => {
+                let formatted = format!("{} {}", &raw[..3], &raw[3..]);
+                if raw == "00000" {
+                    FormattingResult::warning(formatted, "Invalid PSC: 00000")
+                } else {
+                    FormattingResult::success(formatted)
+                }
+            },
+            _ => FormattingResult::error("PSC too long (max 5 digits)"),
+        }
+    }
+}
+
+/// Phone Number Formatter: "1234567890" -> "(123) 456-7890"
+struct PhoneFormatter;
+
+impl CustomFormatter for PhoneFormatter {
+    fn format(&self, raw: &str) -> FormattingResult {
+        if raw.is_empty() {
+            return FormattingResult::success("");
+        }
+        
+        // Only digits allowed
+        if !raw.chars().all(|c| c.is_ascii_digit()) {
+            return FormattingResult::error("Phone must contain only digits");
+        }
+        
+        let len = raw.chars().count();
+        match len {
+            0 => FormattingResult::success(""),
+            1..=3 => FormattingResult::success(format!("({})", raw)),
+            4..=6 => FormattingResult::success(format!("({}) {}", &raw[..3], &raw[3..])),
+            7..=10 => FormattingResult::success(format!("({}) {}-{}", &raw[..3], &raw[3..6], &raw[6..])),
+            10 => {
+                let formatted = format!("({}) {}-{}", &raw[..3], &raw[3..6], &raw[6..]);
+                FormattingResult::success(formatted)
+            },
+            _ => FormattingResult::warning(
+                format!("({}) {}-{}", &raw[..3], &raw[3..6], &raw[6..10]),
+                "Phone too long (extra digits ignored)"
+            ),
+        }
+    }
+}
+
+/// Credit Card Formatter: "1234567890123456" -> "1234 5678 9012 3456"
+struct CreditCardFormatter;
+
+impl CustomFormatter for CreditCardFormatter {
+    fn format(&self, raw: &str) -> FormattingResult {
+        if raw.is_empty() {
+            return FormattingResult::success("");
+        }
+        
+        if !raw.chars().all(|c| c.is_ascii_digit()) {
+            return FormattingResult::error("Card number must contain only digits");
+        }
+        
+        let mut formatted = String::new();
+        for (i, ch) in raw.chars().enumerate() {
+            if i > 0 && i % 4 == 0 {
+                formatted.push(' ');
+            }
+            formatted.push(ch);
+        }
+        
+        let len = raw.chars().count();
+        match len {
+            0..=15 => FormattingResult::warning(formatted, format!("Card incomplete ({}/16 digits)", len)),
+            16 => FormattingResult::success(formatted),
+            _ => FormattingResult::warning(formatted, "Card too long (extra digits shown)"),
+        }
+    }
+}
+
+
+
+
+
+
+/// Date Formatter: "12012024" -> "12/01/2024"
+struct DateFormatter;
+
+impl CustomFormatter for DateFormatter {
+    fn format(&self, raw: &str) -> FormattingResult {
+        if raw.is_empty() {
+            return FormattingResult::success("");
+        }
+        
+        if !raw.chars().all(|c| c.is_ascii_digit()) {
+            return FormattingResult::error("Date must contain only digits");
+        }
+        
+        let len = raw.len();
+        match len {
+            0 => FormattingResult::success(""),
+            1..=2 => FormattingResult::success(raw.to_string()),
+            3..=4 => FormattingResult::success(format!("{}/{}", &raw[..2], &raw[2..])),
+            5..=8 => FormattingResult::success(format!("{}/{}/{}", &raw[..2], &raw[2..4], &raw[4..])),
+            8 => {
+                let month = &raw[..2];
+                let day = &raw[2..4];
+                let year = &raw[4..];
+                
+                // Basic validation
+                let m: u32 = month.parse().unwrap_or(0);
+                let d: u32 = day.parse().unwrap_or(0);
+                
+                if m == 0 || m > 12 {
+                    FormattingResult::warning(
+                        format!("{}/{}/{}", month, day, year),
+                        "Invalid month (01-12)"
+                    )
+                } else if d == 0 || d > 31 {
+                    FormattingResult::warning(
+                        format!("{}/{}/{}", month, day, year),
+                        "Invalid day (01-31)"
+                    )
+                } else {
+                    FormattingResult::success(format!("{}/{}/{}", month, day, year))
+                }
+            },
+            _ => FormattingResult::error("Date too long (MMDDYYYY format)"),
+        }
+    }
+}
+
+// Enhanced demo data with multiple formatter types
+struct MultiFormatterDemoData {
+    fields: Vec<(String, String)>,
+}
+
+impl MultiFormatterDemoData {
+    fn new() -> Self {
+        Self {
+            fields: vec![
+                ("🏁 PSC (01001)".to_string(), "".to_string()),
+                ("📞 Phone (1234567890)".to_string(), "".to_string()),
+                ("💳 Credit Card (16 digits)".to_string(), "".to_string()),
+                ("📅 Date (12012024)".to_string(), "".to_string()),
+                ("📝 Plain Text".to_string(), "".to_string()),
+            ],
+        }
+    }
+}
+
+impl DataProvider for MultiFormatterDemoData {
+    fn field_count(&self) -> usize {
+        self.fields.len()
+    }
+    
+    fn field_name(&self, index: usize) -> &str {
+        &self.fields[index].0
+    }
+    
+    fn field_value(&self, index: usize) -> &str {
+        &self.fields[index].1
+    }
+    
+    fn set_field_value(&mut self, index: usize, value: String) {
+        self.fields[index].1 = value;
+    }
+
+    #[cfg(feature = "validation")]
+    fn validation_config(&self, field_index: usize) -> Option<ValidationConfig> {
+        match field_index {
+            0 => Some(ValidationConfigBuilder::new()
+                .with_custom_formatter(Arc::new(PSCFormatter))
+                .with_max_length(5)
+                .build()),
+            1 => Some(ValidationConfigBuilder::new()
+                .with_custom_formatter(Arc::new(PhoneFormatter))
+                .with_max_length(12)
+                .build()),
+            2 => Some(ValidationConfigBuilder::new()
+                .with_custom_formatter(Arc::new(CreditCardFormatter))
+                .with_max_length(20)
+                .build()),
+            3 => Some(ValidationConfigBuilder::new()
+                .with_custom_formatter(Arc::new(DateFormatter))
+                .with_max_length(8)
+                .build()),
+            4 => Some(ValidationConfigBuilder::new()
+                .with_custom_formatter(Arc::new(DateFormatter))
+                .with_max_length(8)
+                .build()),
+            _ => None, // Plain text field - no formatter
+        }
+    }
+}
+
+// Enhanced demo editor with comprehensive status tracking
+struct EnhancedDemoEditor<D: DataProvider> {
+    editor: FormEditor<D>,
+    debug_message: String,
+    validation_enabled: bool,
+    show_raw_data: bool,
+    show_cursor_details: bool,
+    example_mode: usize,
+}
+
+impl<D: DataProvider> EnhancedDemoEditor<D> {
+    fn new(data_provider: D) -> Self {
+        let mut editor = FormEditor::new(data_provider);
+        editor.set_validation_enabled(true);
+
+        Self {
+            editor,
+            debug_message: "🧩 Enhanced Custom Formatter Demo - Multiple formatters with rich edge cases!".to_string(),
+            validation_enabled: true,
+            show_raw_data: false,
+            show_cursor_details: false,
+            example_mode: 0,
+        }
+    }
+
+    // Field type detection
+    fn current_field_type(&self) -> &'static str {
+        match self.editor.current_field() {
+            0 => "PSC",
+            1 => "Phone",
+            2 => "Credit Card", 
+            3 => "Date",
+            _ => "Plain Text",
+        }
+    }
+
+    fn has_formatter(&self) -> bool {
+        self.editor.current_field() < 5 // First 5 fields have formatters
+    }
+
+    fn get_input_rules(&self) -> &'static str {
+        match self.editor.current_field() {
+            0 => "5 digits only (PSC format)",
+            1 => "10+ digits (US phone format)",
+            2 => "16+ digits (credit card)",
+            3 => "Digits as cents (12345 = $123.45)",
+            4 => "8 digits MMDDYYYY (date format)",
+            _ => "Any text (no formatting)",
+        }
+    }
+
+    fn cycle_example_data(&mut self) {
+        let examples = [
+            // PSC examples
+            vec!["01001", "1234567890", "1234567890123456", "12345", "12012024", "Plain text here"],
+            // Incomplete examples
+            vec!["010", "123", "1234", "123", "1201", "More text"],
+            // Invalid examples (will show error handling)
+            vec!["0abc1", "12a45", "123abc", "abc", "ab01cd", "Special chars!"],
+            // Edge cases
+            vec!["00000", "0000000000", "0000000000000000", "99", "13012024", ""],
+        ];
+        
+        self.example_mode = (self.example_mode + 1) % examples.len();
+        let current_examples = &examples[self.example_mode];
+        
+        for (i, example) in current_examples.iter().enumerate() {
+            if i < self.editor.data_provider().field_count() {
+                self.editor.data_provider_mut().set_field_value(i, example.to_string());
+            }
+        }
+        
+        let mode_names = ["Valid Examples", "Incomplete Input", "Invalid Characters", "Edge Cases"];
+        self.debug_message = format!("📋 Loaded: {}", mode_names[self.example_mode]);
+    }
+
+    // Enhanced status methods
+    fn toggle_validation(&mut self) {
+        self.validation_enabled = !self.validation_enabled;
+        self.editor.set_validation_enabled(self.validation_enabled);
+        self.debug_message = if self.validation_enabled {
+            "✅ Custom Formatters ENABLED".to_string()
+        } else {
+            "❌ Custom Formatters DISABLED".to_string()
+        };
+    }
+
+    fn toggle_raw_data_view(&mut self) {
+        self.show_raw_data = !self.show_raw_data;
+        self.debug_message = if self.show_raw_data {
+            "👁️ Showing RAW data focus".to_string()
+        } else {
+            "✨ Showing FORMATTED display focus".to_string()
+        };
+    }
+
+    fn toggle_cursor_details(&mut self) {
+        self.show_cursor_details = !self.show_cursor_details;
+        self.debug_message = if self.show_cursor_details {
+            "📍 Detailed cursor mapping info ON".to_string()
+        } else {
+            "📍 Detailed cursor mapping info OFF".to_string()
+        };
+    }
+
+    fn get_current_field_analysis(&self) -> (String, String, String, Option<String>) {
+        let raw = self.editor.current_text();
+        let display = self.editor.current_display_text();
+        
+        let status = if raw == display {
+            if self.has_formatter() {
+                if self.mode() == AppMode::Edit {
+                    "Raw (editing)".to_string()
+                } else {
+                    "No formatting needed".to_string()
+                }
+            } else {
+                "No formatter".to_string()
+            }
+        } else {
+            "Custom formatted".to_string()
+        };
+
+        let warning = if self.validation_enabled && self.has_formatter() {
+            // Check if there are any formatting warnings
+            if raw.len() > 0 {
+                match self.editor.current_field() {
+                    0 if raw.len() < 5 => Some(format!("PSC incomplete: {}/5", raw.len())),
+                    1 if raw.len() < 10 => Some(format!("Phone incomplete: {}/10", raw.len())),
+                    2 if raw.len() < 16 => Some(format!("Card incomplete: {}/16", raw.len())),
+                    4 if raw.len() < 8 => Some(format!("Date incomplete: {}/8", raw.len())),
+                    _ => None,
+                }
+            } else {
+                None
+            }
+        } else {
+            None
+        };
+
+        (raw.to_string(), display, status, warning)
+    }
+
+    // Delegate methods with enhanced feedback
+    fn enter_edit_mode(&mut self) {
+        self.editor.enter_edit_mode();
+        let field_type = self.current_field_type();
+        let rules = self.get_input_rules();
+        self.debug_message = format!("✏️ EDITING {} - {}", field_type, rules);
+    }
+
+    fn exit_edit_mode(&mut self) {
+        self.editor.exit_edit_mode();
+        let (raw, display, _, warning) = self.get_current_field_analysis();
+        if let Some(warn) = warning {
+            self.debug_message = format!("🔒 NORMAL - {} | ⚠️ {}", self.current_field_type(), warn);
+        } else if raw != display {
+            self.debug_message = format!("🔒 NORMAL - {} formatted successfully", self.current_field_type());
+        } else {
+            self.debug_message = "🔒 NORMAL MODE".to_string();
+        }
+    }
+
+    fn insert_char(&mut self, ch: char) -> anyhow::Result<()> {
+        let result = self.editor.insert_char(ch);
+        if result.is_ok() {
+            let (raw, display, _, _) = self.get_current_field_analysis();
+            if raw != display && self.validation_enabled {
+                self.debug_message = format!("✏️ '{}' added - Real-time formatting active", ch);
+            } else {
+                self.debug_message = format!("✏️ '{}' added", ch);
+            }
+        }
+        result
+    }
+
+    // Position mapping demo
+    fn show_position_mapping(&mut self) {
+        if !self.has_formatter() {
+            self.debug_message = "📍 No position mapping (plain text field)".to_string();
+            return;
+        }
+
+        let raw_pos = self.editor.cursor_position();
+        let display_pos = self.editor.display_cursor_position();
+        let raw = self.editor.current_text();
+        let display = self.editor.current_display_text();
+        
+        if raw_pos != display_pos {
+            self.debug_message = format!(
+                "🗺️ Position mapping: Raw[{}]='{}' ↔ Display[{}]='{}'",
+                raw_pos,
+                raw.chars().nth(raw_pos).unwrap_or('∅'),
+                display_pos,
+                display.chars().nth(display_pos).unwrap_or('∅')
+            );
+        } else {
+            self.debug_message = format!("📍 Cursor at position {} (no mapping needed)", raw_pos);
+        }
+    }
+
+    // Delegate remaining methods
+    fn mode(&self) -> AppMode { self.editor.mode() }
+    fn current_field(&self) -> usize { self.editor.current_field() }
+    fn cursor_position(&self) -> usize { self.editor.cursor_position() }
+    fn data_provider(&self) -> &D { self.editor.data_provider() }
+    fn data_provider_mut(&mut self) -> &mut D { self.editor.data_provider_mut() }
+    fn ui_state(&self) -> &canvas::EditorState { self.editor.ui_state() }
+    
+    fn move_up(&mut self) { let _ = self.editor.move_up(); }
+    fn move_down(&mut self) { let _ = self.editor.move_down(); }
+    fn move_left(&mut self) { let _ = self.editor.move_left(); }
+    fn move_right(&mut self) { let _ = self.editor.move_right(); }
+    fn delete_backward(&mut self) -> anyhow::Result<()> { self.editor.delete_backward() }
+    fn delete_forward(&mut self) -> anyhow::Result<()> { self.editor.delete_forward() }
+    fn next_field(&mut self) { let _ = self.editor.next_field(); }
+    fn prev_field(&mut self) { let _ = self.editor.prev_field(); }
+}
+
+// Enhanced key handling
+fn handle_key_press(
+    key: KeyCode,
+    modifiers: KeyModifiers,
+    editor: &mut EnhancedDemoEditor<MultiFormatterDemoData>,
+) -> anyhow::Result<bool> {
+    let mode = editor.mode();
+
+    // Quit
+    if matches!(key, KeyCode::F(10)) || 
+       (key == KeyCode::Char('q') && modifiers.contains(KeyModifiers::CONTROL)) ||
+       (key == KeyCode::Char('c') && modifiers.contains(KeyModifiers::CONTROL)) {
+        return Ok(false);
+    }
+
+    match (mode, key, modifiers) {
+        // Mode transitions
+        (AppMode::ReadOnly, KeyCode::Char('i'), _) => editor.enter_edit_mode(),
+        (AppMode::ReadOnly, KeyCode::Char('a'), _) => {
+            editor.editor.enter_append_mode();
+            editor.debug_message = format!("✏️ APPEND {} - {}", editor.current_field_type(), editor.get_input_rules());
+        },
+        (_, KeyCode::Esc, _) => editor.exit_edit_mode(),
+
+        // Enhanced demo features
+        (AppMode::ReadOnly, KeyCode::Char('e'), _) => editor.cycle_example_data(),
+        (AppMode::ReadOnly, KeyCode::Char('r'), _) => editor.toggle_raw_data_view(),
+        (AppMode::ReadOnly, KeyCode::Char('c'), _) => editor.toggle_cursor_details(),
+        (AppMode::ReadOnly, KeyCode::Char('m'), _) => editor.show_position_mapping(),
+        (AppMode::ReadOnly, KeyCode::F(1), _) => editor.toggle_validation(),
+
+        // Movement
+        (_, KeyCode::Up, _) | (AppMode::ReadOnly, KeyCode::Char('k'), _) => editor.move_up(),
+        (_, KeyCode::Down, _) | (AppMode::ReadOnly, KeyCode::Char('j'), _) => editor.move_down(),
+        (_, KeyCode::Left, _) | (AppMode::ReadOnly, KeyCode::Char('h'), _) => editor.move_left(),
+        (_, KeyCode::Right, _) | (AppMode::ReadOnly, KeyCode::Char('l'), _) => editor.move_right(),
+        (_, KeyCode::Tab, _) => editor.next_field(),
+        (_, KeyCode::BackTab, _) => editor.prev_field(),
+
+        // Editing
+        (AppMode::Edit, KeyCode::Char(c), m) if !m.contains(KeyModifiers::CONTROL) => {
+            editor.insert_char(c)?;
+        },
+        (AppMode::Edit, KeyCode::Backspace, _) => { editor.delete_backward()?; },
+        (AppMode::Edit, KeyCode::Delete, _) => { editor.delete_forward()?; },
+
+        // Field analysis
+        (AppMode::ReadOnly, KeyCode::Char('?'), _) => {
+            let (raw, display, status, warning) = editor.get_current_field_analysis();
+            let warning_text = warning.map(|w| format!(" ⚠️ {}", w)).unwrap_or_default();
+            editor.debug_message = format!(
+                "🔍 Field {}: {} | Raw: '{}' | Display: '{}'{}", 
+                editor.current_field() + 1, status, raw, display, warning_text
+            );
+        },
+
+        _ => {}
+    }
+
+    Ok(true)
+}
+
+fn run_app<B: Backend>(
+    terminal: &mut Terminal<B>,
+    mut editor: EnhancedDemoEditor<MultiFormatterDemoData>,
+) -> io::Result<()> {
+    loop {
+        terminal.draw(|f| ui(f, &editor))?;
+
+        if let Event::Key(key) = event::read()? {
+            match handle_key_press(key.code, key.modifiers, &mut editor) {
+                Ok(should_continue) => {
+                    if !should_continue {
+                        break;
+                    }
+                }
+                Err(e) => {
+                    editor.debug_message = format!("❌ Error: {}", e);
+                }
+            }
+        }
+    }
+    Ok(())
+}
+
+fn ui(f: &mut Frame, editor: &EnhancedDemoEditor<MultiFormatterDemoData>) {
+    let chunks = Layout::default()
+        .direction(Direction::Vertical)
+        .constraints([Constraint::Min(8), Constraint::Length(18)])
+        .split(f.area());
+
+    render_canvas_default(f, chunks[0], &editor.editor);
+    render_enhanced_status(f, chunks[1], editor);
+}
+
+fn render_enhanced_status(
+    f: &mut Frame,
+    area: Rect,
+    editor: &EnhancedDemoEditor<MultiFormatterDemoData>,
+) {
+    let chunks = Layout::default()
+        .direction(Direction::Vertical)
+        .constraints([
+            Constraint::Length(3),  // Status bar
+            Constraint::Length(6),  // Current field analysis
+            Constraint::Length(9),  // Help
+        ])
+        .split(area);
+
+    // Status bar
+    let mode_text = match editor.mode() {
+        AppMode::Edit => "INSERT",
+        AppMode::ReadOnly => "NORMAL",
+        _ => "OTHER",
+    };
+
+    let formatter_count = (0..editor.data_provider().field_count())
+        .filter(|&i| editor.data_provider().validation_config(i).is_some())
+        .count();
+
+    let status_text = format!(
+        "-- {} -- {} | Formatters: {}/{} active | View: {}{}",
+        mode_text,
+        editor.debug_message,
+        formatter_count,
+        editor.data_provider().field_count(),
+        if editor.show_raw_data { "RAW" } else { "DISPLAY" },
+        if editor.show_cursor_details { " | CURSOR+" } else { "" }
+    );
+
+    let status = Paragraph::new(Line::from(Span::raw(status_text)))
+        .block(Block::default().borders(Borders::ALL).title("🧩 Enhanced Custom Formatter Demo"));
+
+    f.render_widget(status, chunks[0]);
+
+    // Current field analysis
+    let (raw, display, status, warning) = editor.get_current_field_analysis();
+    let field_name = editor.data_provider().field_name(editor.current_field());
+    let field_type = editor.current_field_type();
+    
+    let mut analysis_lines = vec![
+        format!("📝 Current: {} ({})", field_name, field_type),
+        format!("🔧 Status: {}", status),
+    ];
+
+    if editor.show_raw_data || editor.mode() == AppMode::Edit {
+        analysis_lines.push(format!("💾 Raw Data: '{}'", raw));
+        analysis_lines.push(format!("✨ Display: '{}'", display));
+    } else {
+        analysis_lines.push(format!("✨ User Sees: '{}'", display));
+        analysis_lines.push(format!("💾 Stored As: '{}'", raw));
+    }
+
+    if editor.show_cursor_details {
+        analysis_lines.push(format!(
+            "📍 Cursor: Raw[{}] → Display[{}]",
+            editor.cursor_position(),
+            editor.editor.display_cursor_position()
+        ));
+    }
+
+    if let Some(ref warn) = warning {
+        analysis_lines.push(format!("⚠️ Warning: {}", warn));
+    }
+
+    let analysis_color = if warning.is_some() {
+        Color::Yellow
+    } else if raw != display && editor.validation_enabled {
+        Color::Green
+    } else {
+        Color::Gray
+    };
+
+    let analysis = Paragraph::new(analysis_lines.join("\n"))
+        .block(Block::default().borders(Borders::ALL).title("🔍 Field Analysis"))
+        .style(Style::default().fg(analysis_color))
+        .wrap(Wrap { trim: true });
+
+    f.render_widget(analysis, chunks[1]);
+
+    // Enhanced help
+    let help_text = match editor.mode() {
+        AppMode::ReadOnly => {
+            "🧩 ENHANCED CUSTOM FORMATTER DEMO\n\
+             \n\
+             Try these formatters:
+             • PSC: 01001 → 010 01 | Phone: 1234567890 → (123) 456-7890 | Card: 1234567890123456 → 1234 5678 9012 3456
+             • Date: 12012024 → 12/01/2024 | Plain: no formatting
+             \n\
+             Commands: i=insert, e=cycle examples, r=toggle raw/display, c=cursor details, m=position mapping\n\
+             Movement: hjkl/arrows, Tab=next field, ?=analyze current field, F1=toggle formatters\n\
+             Ctrl+C/F10=quit"
+        }
+        AppMode::Edit => {
+            "✏️ INSERT MODE - Real-time formatting as you type!\n\
+             \n\
+             Current field rules: {}\n\
+             • Raw input is authoritative (what gets stored)\n\
+             • Display formatting updates in real-time (what users see)\n\
+             • Cursor position is mapped between raw and display\n\
+             \n\
+             Esc=normal mode, arrows=navigate, Backspace/Del=delete"
+        }
+        _ => "🧩 Enhanced Custom Formatter Demo"
+    };
+
+    let formatted_help = if editor.mode() == AppMode::Edit {
+        help_text.replace("{}", editor.get_input_rules())
+    } else {
+        help_text.to_string()
+    };
+
+    let help = Paragraph::new(formatted_help)
+        .block(Block::default().borders(Borders::ALL).title("🚀 Enhanced Features & Commands"))
+        .style(Style::default().fg(Color::Gray))
+        .wrap(Wrap { trim: true });
+
+    f.render_widget(help, chunks[2]);
+}
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    println!("🧩 Enhanced Canvas Custom Formatter Demo (Feature 4)");
+    println!("✅ validation feature: ENABLED");
+    println!("✅ gui feature: ENABLED");
+    println!("🧩 Enhanced features:");
+    println!("   • 5 different custom formatters with edge cases");
+    println!("   • Real-time format preview and validation");
+    println!("   • Advanced cursor position mapping");
+    println!("   • Comprehensive error handling and warnings");
+    println!("   • Raw vs formatted data separation demos");
+    println!();
+
+    enable_raw_mode()?;
+    let mut stdout = io::stdout();
+    execute!(stdout, EnterAlternateScreen, EnableMouseCapture)?;
+    let backend = CrosstermBackend::new(stdout);
+    let mut terminal = Terminal::new(backend)?;
+
+    let data = MultiFormatterDemoData::new();
+    let editor = EnhancedDemoEditor::new(data);
+
+    let res = run_app(&mut terminal, editor);
+
+    disable_raw_mode()?;
+    execute!(terminal.backend_mut(), LeaveAlternateScreen, DisableMouseCapture)?;
+    terminal.show_cursor()?;
+
+    if let Err(err) = res {
+        println!("{:?}", err);
+    }
+
+    println!("🧩 Enhanced custom formatter demo completed!");
+    println!("🏆 You experienced comprehensive custom formatting with:");
+    println!("   • Multiple formatter types (PSC, Phone, Credit Card, Date)");
+    println!("   • Edge case handling (incomplete, invalid, overflow)");
+    println!("   • Real-time format preview and cursor mapping");
+    println!("   • Clear separation between raw business data and display formatting");
+    Ok(())
+}

canvas/src/canvas/gui.rs

@@ -52,7 +52,16 @@ pub fn render_canvas_with_highlight<T: CanvasTheme, D: DataProvider>(
 
     for i in 0..field_count {
         fields.push(data_provider.field_name(i));
-        inputs.push(data_provider.field_value(i).to_string());
+
+        // Use editor-provided effective display text per field (Feature 4/mask aware)
+        #[cfg(feature = "validation")]
+        {
+            inputs.push(editor.display_text_for_field(i));
+        }
+        #[cfg(not(feature = "validation"))]
+        {
+            inputs.push(data_provider.field_value(i).to_string());
+        }
     }
 
     let current_field_idx = ui_state.current_field();
@@ -66,13 +75,42 @@ pub fn render_canvas_with_highlight<T: CanvasTheme, D: DataProvider>(
         &inputs,
         theme,
         is_edit_mode,
-        highlight_state, // Now using the actual highlight state!
-        ui_state.cursor_position(),
+        highlight_state,
+        editor.display_cursor_position(), // Use display cursor position for masks
         false, // TODO: track unsaved changes in editor
         |i| {
-            data_provider.display_value(i).unwrap_or(data_provider.field_value(i)).to_string()
+            // Get display value for field i using editor logic (Feature 4 + masks)
+            #[cfg(feature = "validation")]
+            {
+                editor.display_text_for_field(i)
+            }
+            #[cfg(not(feature = "validation"))]
+            {
+                data_provider.field_value(i).to_string()
+            }
+        },
+        |i| {
+            // Check if field has display override (custom formatter or mask)
+            #[cfg(feature = "validation")]
+            {
+                editor.ui_state().validation_state().get_field_config(i)
+                    .map(|cfg| {
+                        // Formatter takes precedence; if present, it's a display override
+                        #[allow(unused_mut)]
+                        let mut has_override = false;
+                        #[cfg(feature = "validation")]
+                        {
+                            has_override = cfg.custom_formatter.is_some();
+                        }
+                        has_override || cfg.display_mask.is_some()
+                    })
+                    .unwrap_or(false)
+            }
+            #[cfg(not(feature = "validation"))]
+            {
+                false
+            }
         },
-        |i| data_provider.display_value(i).is_some(),
     )
 }
 
@@ -245,7 +283,7 @@ fn apply_highlighting<'a, T: CanvasTheme>(
     current_cursor_pos: usize,
     highlight_state: &HighlightState,
     theme: &T,
-    is_active: bool,
+    _is_active: bool,
 ) -> Line<'a> {
     let text_len = text.chars().count();
 
@@ -257,10 +295,10 @@ fn apply_highlighting<'a, T: CanvasTheme>(
             ))
         }
         HighlightState::Characterwise { anchor } => {
-            apply_characterwise_highlighting(text, text_len, field_index, current_field_idx, current_cursor_pos, anchor, theme, is_active)
+            apply_characterwise_highlighting(text, text_len, field_index, current_field_idx, current_cursor_pos, anchor, theme, _is_active)
         }
         HighlightState::Linewise { anchor_line } => {
-            apply_linewise_highlighting(text, field_index, current_field_idx, anchor_line, theme, is_active)
+            apply_linewise_highlighting(text, field_index, current_field_idx, anchor_line, theme, _is_active)
         }
     }
 }
@@ -275,7 +313,7 @@ fn apply_characterwise_highlighting<'a, T: CanvasTheme>(
     current_cursor_pos: usize,
     anchor: &(usize, usize),
     theme: &T,
-    is_active: bool,
+    _is_active: bool,
 ) -> Line<'a> {
     let (anchor_field, anchor_char) = *anchor;
     let start_field = min(anchor_field, *current_field_idx);
@@ -378,7 +416,7 @@ fn apply_linewise_highlighting<'a, T: CanvasTheme>(
     current_field_idx: &usize,
     anchor_line: &usize,
     theme: &T,
-    is_active: bool,
+    _is_active: bool,
 ) -> Line<'a> {
     let start_field = min(*anchor_line, *current_field_idx);
     let end_field = max(*anchor_line, *current_field_idx);
@@ -411,13 +449,15 @@ fn set_cursor_position(
     current_cursor_pos: usize,
     has_display_override: bool,
 ) {
-    let cursor_x = if has_display_override {
-        field_rect.x + text.chars().count() as u16
-    } else {
-        field_rect.x + current_cursor_pos as u16
-    };
+    // BUG FIX: Use the correct display cursor position, not end of text
+    let cursor_x = field_rect.x + current_cursor_pos as u16;
     let cursor_y = field_rect.y;
-    f.set_cursor_position((cursor_x, cursor_y));
+    
+    // SAFETY: Ensure cursor doesn't go beyond field bounds
+    let max_cursor_x = field_rect.x + field_rect.width.saturating_sub(1);
+    let safe_cursor_x = cursor_x.min(max_cursor_x);
+    
+    f.set_cursor_position((safe_cursor_x, cursor_y));
 }
 
 /// Set default theme if custom not specified

canvas/src/editor.rs

@@ -3,8 +3,7 @@
 
 #[cfg(feature = "cursor-style")]
 use crate::canvas::CursorManager;
-#[cfg(feature = "cursor-style")]
-use crossterm;
+
 
 use anyhow::Result;
 use crate::canvas::state::EditorState;
@@ -31,16 +30,16 @@ impl<D: DataProvider> FormEditor<D> {
             data_provider,
             suggestions: Vec::new(),
         };
-        
+
         // Initialize validation configurations if validation feature is enabled
         #[cfg(feature = "validation")]
         {
             editor.initialize_validation();
         }
-        
+
         editor
     }
-    
+
     /// Initialize validation configurations from data provider
     #[cfg(feature = "validation")]
     fn initialize_validation(&mut self) {
@@ -86,11 +85,120 @@ impl<D: DataProvider> FormEditor<D> {
         }
     }
 
+    /// Get current field text for display.
+    ///
+    /// Policies:
+    /// - Feature 4 (custom formatter):
+    ///   - While editing the focused field: ALWAYS show raw (no custom formatting).
+    ///   - When not editing the field: show formatted (fallback to raw on error).
+    /// - Mask-only fields: mask applies even in Edit mode (preserve legacy behavior).
+    /// - Otherwise: raw.
+    #[cfg(feature = "validation")]
+    pub fn current_display_text(&self) -> String {
+        let field_index = self.ui_state.current_field;
+        let raw = if field_index < self.data_provider.field_count() {
+            self.data_provider.field_value(field_index)
+        } else {
+            ""
+        };
+
+        if let Some(cfg) = self.ui_state.validation.get_field_config(field_index) {
+            // 1) Mask-only fields: mask applies even in Edit (legacy behavior)
+            if cfg.custom_formatter.is_none() {
+                if let Some(mask) = &cfg.display_mask {
+                    return mask.apply_to_display(raw);
+                }
+            }
+
+            // 2) Feature 4 fields: raw while editing, formatted otherwise
+            if cfg.custom_formatter.is_some() {
+                if matches!(self.ui_state.current_mode, AppMode::Edit) {
+                    return raw.to_string();
+                }
+                if let Some((formatted, _mapper, _warning)) = cfg.run_custom_formatter(raw) {
+                    return formatted;
+                }
+            }
+
+            // 3) Fallback to mask if present (when formatter didn't produce output)
+            if let Some(mask) = &cfg.display_mask {
+                return mask.apply_to_display(raw);
+            }
+        }
+
+        raw.to_string()
+    }
+
     /// Get reference to UI state for rendering
     pub fn ui_state(&self) -> &EditorState {
         &self.ui_state
     }
 
+    /// Set external validation state for a field (Feature 5)
+    #[cfg(feature = "validation")]
+    pub fn set_external_validation(
+        &mut self,
+        field_index: usize,
+        state: crate::validation::ExternalValidationState,
+    ) {
+        self.ui_state
+            .validation
+            .set_external_validation(field_index, state);
+    }
+
+    /// Clear external validation state for a field (Feature 5)
+    #[cfg(feature = "validation")]
+    pub fn clear_external_validation(&mut self, field_index: usize) {
+        self.ui_state.validation.clear_external_validation(field_index);
+    }
+
+    /// Get effective display text for any field index.
+    ///
+    /// Policies:
+    /// - Feature 4 fields (with custom formatter):
+    ///   - If the field is currently focused AND in Edit mode: return raw (no formatting).
+    ///   - Otherwise: return formatted (fallback to raw on error).
+    /// - Mask-only fields: mask applies regardless of mode (legacy behavior).
+    /// - Otherwise: raw.
+    #[cfg(feature = "validation")]
+    pub fn display_text_for_field(&self, field_index: usize) -> String {
+        let raw = if field_index < self.data_provider.field_count() {
+            self.data_provider.field_value(field_index)
+        } else {
+            ""
+        };
+
+        if let Some(cfg) = self.ui_state.validation.get_field_config(field_index) {
+            // Mask-only fields: mask applies even in Edit mode
+            if cfg.custom_formatter.is_none() {
+                if let Some(mask) = &cfg.display_mask {
+                    return mask.apply_to_display(raw);
+                }
+            }
+
+            // Feature 4 fields:
+            if cfg.custom_formatter.is_some() {
+                // Focused + Edit -> raw
+                if field_index == self.ui_state.current_field
+                    && matches!(self.ui_state.current_mode, AppMode::Edit)
+                {
+                    return raw.to_string();
+                }
+                // Not editing -> formatted
+                if let Some((formatted, _mapper, _warning)) = cfg.run_custom_formatter(raw) {
+                    return formatted;
+                }
+            }
+
+            // Fallback to mask if present (in case formatter didn't return output)
+            if let Some(mask) = &cfg.display_mask {
+                return mask.apply_to_display(raw);
+            }
+        }
+
+        raw.to_string()
+    }
+
     /// Get reference to data provider for rendering
     pub fn data_provider(&self) -> &D {
         &self.data_provider
@@ -100,20 +208,20 @@ impl<D: DataProvider> FormEditor<D> {
     pub fn suggestions(&self) -> &[SuggestionItem] {
         &self.suggestions
     }
-    
+
     /// Get validation state (for user's business logic)
     /// Only available when the 'validation' feature is enabled
     #[cfg(feature = "validation")]
     pub fn validation_state(&self) -> &crate::validation::ValidationState {
         self.ui_state.validation_state()
     }
-    
+
     /// Get validation result for current field
     #[cfg(feature = "validation")]
     pub fn current_field_validation(&self) -> Option<&crate::validation::ValidationResult> {
         self.ui_state.validation.get_field_result(self.ui_state.current_field)
     }
-    
+
     /// Get validation result for specific field
     #[cfg(feature = "validation")]
     pub fn field_validation(&self, field_index: usize) -> Option<&crate::validation::ValidationResult> {
@@ -124,31 +232,69 @@ impl<D: DataProvider> FormEditor<D> {
     // SYNC OPERATIONS: No async needed for basic editing
     // ===================================================================
 
-    /// Handle character insertion
+    /// Handle character insertion with proper mask/limit coordination
     pub fn insert_char(&mut self, ch: char) -> Result<()> {
         if self.ui_state.current_mode != AppMode::Edit {
             return Ok(()); // Ignore in non-edit modes
         }
 
         let field_index = self.ui_state.current_field;
-        let cursor_pos = self.ui_state.cursor_pos;
+        let raw_cursor_pos = self.ui_state.cursor_pos;
+        let current_raw_text = self.data_provider.field_value(field_index);
 
-        // Get current text from user
-        let current_text = self.data_provider.field_value(field_index);
+        // 🔥 CRITICAL FIX 1: Check mask constraints FIRST
+        #[cfg(feature = "validation")]
+        {
+            if let Some(cfg) = self.ui_state.validation.get_field_config(field_index) {
+                if let Some(mask) = &cfg.display_mask {
+                    // Get display cursor position
+                    let display_cursor_pos = mask.raw_pos_to_display_pos(raw_cursor_pos);
+                    
+                    // ❌ PREVENT BUG: Reject input if cursor is beyond mask pattern
+                    if display_cursor_pos >= mask.pattern().len() {
+                        tracing::debug!(
+                            "Character insertion rejected: cursor beyond mask pattern length"
+                        );
+                        return Ok(()); // Silently reject - user can't type beyond mask
+                    }
+                    
+                    // ❌ PREVENT BUG: Reject input if cursor is on a separator position
+                    if !mask.is_input_position(display_cursor_pos) {
+                        tracing::debug!(
+                            "Character insertion rejected: cursor on separator position {}", 
+                            display_cursor_pos
+                        );
+                        return Ok(()); // Silently reject - can't type on separators
+                    }
+                    
+                    // ❌ PREVENT BUG: Check if we're at max input positions for this mask
+                    let input_char_count = (0..mask.pattern().len())
+                        .filter(|&pos| mask.is_input_position(pos))
+                        .count();
+                    
+                    if current_raw_text.len() >= input_char_count {
+                        tracing::debug!(
+                            "Character insertion rejected: mask pattern full ({} input positions)", 
+                            input_char_count
+                        );
+                        return Ok(()); // Silently reject - mask is full
+                    }
+                }
+            }
+        }
 
-        // Validate character insertion if validation is enabled
+        // 🔥 CRITICAL FIX 2: Validate character insertion with mask awareness
         #[cfg(feature = "validation")]
         {
             let validation_result = self.ui_state.validation.validate_char_insertion(
                 field_index,
-                current_text,
-                cursor_pos,
+                current_raw_text,
+                raw_cursor_pos,
                 ch,
             );
-            
+
             // Reject input if validation failed with error
             if !validation_result.is_acceptable() {
-                // Log validation failure for debugging
                 tracing::debug!(
                     "Character insertion rejected for field {}: {:?}",
                     field_index,
@@ -158,38 +304,130 @@ impl<D: DataProvider> FormEditor<D> {
             }
         }
 
-        // Insert character
-        let mut new_text = current_text.to_string();
-        new_text.insert(cursor_pos, ch);
+        // 🔥 CRITICAL FIX 3: Validate the insertion won't break display/limit coordination
+        let new_raw_text = {
+            let mut temp = current_raw_text.to_string();
+            temp.insert(raw_cursor_pos, ch);
+            temp
+        };
 
-        // Update user's data
-        self.data_provider.set_field_value(field_index, new_text);
+        #[cfg(feature = "validation")]
+        {
+            if let Some(cfg) = self.ui_state.validation.get_field_config(field_index) {
+                // Check character limits on the new raw text
+                if let Some(limits) = &cfg.character_limits {
+                    if let Some(result) = limits.validate_content(&new_raw_text) {
+                        if !result.is_acceptable() {
+                            tracing::debug!(
+                                "Character insertion rejected: would exceed character limits"
+                            );
+                            return Ok(()); // Silently reject - would exceed limits
+                        }
+                    }
+                }
+                
+                // Check that mask can handle the new raw text length
+                if let Some(mask) = &cfg.display_mask {
+                    let input_positions = (0..mask.pattern().len())
+                        .filter(|&pos| mask.is_input_position(pos))
+                        .count();
+                    
+                    if new_raw_text.len() > input_positions {
+                        tracing::debug!(
+                            "Character insertion rejected: raw text length {} exceeds mask input positions {}", 
+                            new_raw_text.len(), 
+                            input_positions
+                        );
+                        return Ok(()); // Silently reject - mask can't handle this length
+                    }
+                }
+            }
+        }
 
-        // Update library's UI state
-        self.ui_state.cursor_pos += 1;
+        // ✅ ALL CHECKS PASSED: Safe to insert character
+        self.data_provider.set_field_value(field_index, new_raw_text);
+
+        // 🔥 CRITICAL FIX 4: Update cursor position correctly for mask context
+        #[cfg(feature = "validation")]
+        {
+            if let Some(cfg) = self.ui_state.validation.get_field_config(field_index) {
+                if let Some(mask) = &cfg.display_mask {
+                    // Move to next input position, skipping separators
+                    let new_raw_pos = raw_cursor_pos + 1;
+                    let display_pos = mask.raw_pos_to_display_pos(new_raw_pos);
+                    let next_input_pos = mask.next_input_position(display_pos);
+                    let next_raw_pos = mask.display_pos_to_raw_pos(next_input_pos);
+                    
+                    self.ui_state.cursor_pos = next_raw_pos;
+                    self.ui_state.ideal_cursor_column = next_raw_pos;
+                    return Ok(());
+                }
+            }
+        }
+
+        // No mask: simple increment
+        self.ui_state.cursor_pos = raw_cursor_pos + 1;
         self.ui_state.ideal_cursor_column = self.ui_state.cursor_pos;
 
         Ok(())
     }
 
-    /// Handle cursor movement
+    /// Handle cursor movement left - skips mask separator positions
     pub fn move_left(&mut self) {
-        if self.ui_state.cursor_pos > 0 {
-            self.ui_state.cursor_pos -= 1;
-            self.ui_state.ideal_cursor_column = self.ui_state.cursor_pos;
+        if self.ui_state.cursor_pos == 0 {
+            return;
         }
+
+        let field_index = self.ui_state.current_field;
+        let mut new_pos = self.ui_state.cursor_pos - 1;
+
+        // Skip mask separator positions if configured
+        #[cfg(feature = "validation")]
+        if let Some(cfg) = self.ui_state.validation.get_field_config(field_index) {
+            if let Some(mask) = &cfg.display_mask {
+                // Convert to display position, find previous input position, convert back
+                let display_pos = mask.raw_pos_to_display_pos(new_pos);
+                if let Some(prev_input_display_pos) = mask.prev_input_position(display_pos) {
+                    new_pos = mask.display_pos_to_raw_pos(prev_input_display_pos);
+                }
+            }
+        }
+
+        self.ui_state.cursor_pos = new_pos;
+        self.ui_state.ideal_cursor_column = self.ui_state.cursor_pos;
     }
 
+    /// Handle cursor movement right - skips mask separator positions  
     pub fn move_right(&mut self) {
         let current_text = self.current_text();
-        let max_pos = if self.ui_state.current_mode == AppMode::Edit {
-            current_text.len() // Edit mode: can go past end
+        let is_edit_mode = self.ui_state.current_mode == AppMode::Edit;
+        let max_pos = if is_edit_mode {
+            current_text.len()
         } else {
-            current_text.len().saturating_sub(1) // ReadOnly: stay in bounds
+            current_text.len().saturating_sub(1)
         };
 
-        if self.ui_state.cursor_pos < max_pos {
-            self.ui_state.cursor_pos += 1;
+        if self.ui_state.cursor_pos >= max_pos {
+            return;
+        }
+
+        let field_index = self.ui_state.current_field;
+        let mut new_pos = self.ui_state.cursor_pos + 1;
+
+        // Skip mask separator positions if configured
+        #[cfg(feature = "validation")]
+        if let Some(cfg) = self.ui_state.validation.get_field_config(field_index) {
+            if let Some(mask) = &cfg.display_mask {
+                // Convert to display position, find next input position, convert back
+                let display_pos = mask.raw_pos_to_display_pos(new_pos);
+                let next_input_display_pos = mask.next_input_position(display_pos);
+                new_pos = mask.display_pos_to_raw_pos(next_input_display_pos);
+                new_pos = new_pos.min(max_pos);
+            }
+        }
+
+        if new_pos <= max_pos {
+            self.ui_state.cursor_pos = new_pos;
             self.ui_state.ideal_cursor_column = self.ui_state.cursor_pos;
         }
     }
@@ -198,7 +436,7 @@ impl<D: DataProvider> FormEditor<D> {
     pub fn move_to_next_field(&mut self) {
         let field_count = self.data_provider.field_count();
         let next_field = (self.ui_state.current_field + 1) % field_count;
-        
+
         // Validate current field content before moving if validation is enabled
         #[cfg(feature = "validation")]
         {
@@ -210,7 +448,7 @@ impl<D: DataProvider> FormEditor<D> {
             // Note: We don't prevent field switching on validation failure,
             // just record the validation state
         }
-        
+
         self.ui_state.move_to_field(next_field, field_count);
 
         // Clamp cursor to new field
@@ -271,31 +509,31 @@ impl<D: DataProvider> FormEditor<D> {
     // ===================================================================
     // VALIDATION METHODS (only available with validation feature)
     // ===================================================================
-    
+
     /// Enable or disable validation
     #[cfg(feature = "validation")]
     pub fn set_validation_enabled(&mut self, enabled: bool) {
         self.ui_state.validation.set_enabled(enabled);
     }
-    
+
     /// Check if validation is enabled
     #[cfg(feature = "validation")]
     pub fn is_validation_enabled(&self) -> bool {
         self.ui_state.validation.is_enabled()
     }
-    
+
     /// Set validation configuration for a specific field
     #[cfg(feature = "validation")]
     pub fn set_field_validation(&mut self, field_index: usize, config: crate::validation::ValidationConfig) {
         self.ui_state.validation.set_field_config(field_index, config);
     }
-    
+
     /// Remove validation configuration for a specific field
     #[cfg(feature = "validation")]
     pub fn remove_field_validation(&mut self, field_index: usize) {
         self.ui_state.validation.remove_field_config(field_index);
     }
-    
+
     /// Manually validate current field content
     #[cfg(feature = "validation")]
     pub fn validate_current_field(&mut self) -> crate::validation::ValidationResult {
@@ -303,7 +541,7 @@ impl<D: DataProvider> FormEditor<D> {
         let current_text = self.current_text().to_string();
         self.ui_state.validation.validate_field_content(field_index, &current_text)
     }
-    
+
     /// Manually validate specific field content
     #[cfg(feature = "validation")]
     pub fn validate_field(&mut self, field_index: usize) -> Option<crate::validation::ValidationResult> {
@@ -314,26 +552,26 @@ impl<D: DataProvider> FormEditor<D> {
             None
         }
     }
-    
+
     /// Clear validation results for all fields
     #[cfg(feature = "validation")]
     pub fn clear_validation_results(&mut self) {
         self.ui_state.validation.clear_all_results();
     }
-    
+
     /// Get validation summary for all fields
     #[cfg(feature = "validation")]
     pub fn validation_summary(&self) -> crate::validation::ValidationSummary {
         self.ui_state.validation.summary()
     }
-    
+
     /// Check if field switching is allowed from current field
     #[cfg(feature = "validation")]
     pub fn can_switch_fields(&self) -> bool {
         let current_text = self.current_text();
         self.ui_state.validation.allows_field_switch(self.ui_state.current_field, current_text)
     }
-    
+
     /// Get reason why field switching is blocked (if any)
     #[cfg(feature = "validation")]
     pub fn field_switch_block_reason(&self) -> Option<String> {
@@ -402,7 +640,7 @@ impl<D: DataProvider> FormEditor<D> {
                 // Close autocomplete
                 self.ui_state.deactivate_autocomplete();
                 self.suggestions.clear();
-                
+
                 // Validate the new content if validation is enabled
                 #[cfg(feature = "validation")]
                 {
@@ -648,7 +886,7 @@ impl<D: DataProvider> FormEditor<D> {
             self.data_provider.set_field_value(field_index, current_text.clone());
             self.ui_state.cursor_pos -= 1;
             self.ui_state.ideal_cursor_column = self.ui_state.cursor_pos;
-            
+
             // Validate the new content if validation is enabled
             #[cfg(feature = "validation")]
             {
@@ -674,7 +912,7 @@ impl<D: DataProvider> FormEditor<D> {
         if self.ui_state.cursor_pos < current_text.len() {
             current_text.remove(self.ui_state.cursor_pos);
             self.data_provider.set_field_value(field_index, current_text.clone());
-            
+
             // Validate the new content if validation is enabled
             #[cfg(feature = "validation")]
             {
@@ -700,7 +938,7 @@ impl<D: DataProvider> FormEditor<D> {
                 }
             }
         }
-        
+
         // Adjust cursor position when transitioning from edit to normal mode
         let current_text = self.current_text();
         if !current_text.is_empty() {
@@ -751,7 +989,7 @@ impl<D: DataProvider> FormEditor<D> {
         // Reset cursor to start of field
         self.ui_state.cursor_pos = 0;
         self.ui_state.ideal_cursor_column = 0;
-        
+
         // Validate the new content if validation is enabled
         #[cfg(feature = "validation")]
         {
@@ -771,7 +1009,7 @@ impl<D: DataProvider> FormEditor<D> {
                 self.ui_state.cursor_pos = 0;
                 self.ui_state.ideal_cursor_column = 0;
             }
-            
+
             // Validate the new content if validation is enabled
             #[cfg(feature = "validation")]
             {
@@ -812,24 +1050,38 @@ impl<D: DataProvider> FormEditor<D> {
         self.ui_state.ideal_cursor_column = clamped_pos;
     }
 
-    /// Get cursor position for display (respects mode-specific positioning rules)
+    /// Get cursor position for display (maps raw cursor to display position with formatter/mask)
     pub fn display_cursor_position(&self) -> usize {
         let current_text = self.current_text();
-
-        match self.ui_state.current_mode {
-            AppMode::Edit => {
-                // Edit mode: cursor can be past end of text
-                self.ui_state.cursor_pos.min(current_text.len())
-            }
+        let raw_pos = match self.ui_state.current_mode {
+            AppMode::Edit => self.ui_state.cursor_pos.min(current_text.len()),
             _ => {
-                // Normal/other modes: cursor must be on a character
                 if current_text.is_empty() {
                     0
                 } else {
                     self.ui_state.cursor_pos.min(current_text.len().saturating_sub(1))
                 }
             }
+        };
+
+        #[cfg(feature = "validation")]
+        {
+            let field_index = self.ui_state.current_field;
+            if let Some(cfg) = self.ui_state.validation.get_field_config(field_index) {
+                // Only apply custom formatter cursor mapping when NOT editing
+                if !matches!(self.ui_state.current_mode, AppMode::Edit) {
+                    if let Some((formatted, mapper, _warning)) = cfg.run_custom_formatter(current_text) {
+                        return mapper.raw_to_formatted(current_text, &formatted, raw_pos);
+                    }
+                }
+                // Fallback to display mask
+                if let Some(mask) = &cfg.display_mask {
+                    return mask.raw_pos_to_display_pos(self.ui_state.cursor_pos);
+                }
+            }
         }
+
+        self.ui_state.cursor_pos
     }
 
     /// Cleanup cursor style (call this when shutting down)
@@ -914,12 +1166,12 @@ impl<D: DataProvider> FormEditor<D> {
     }
 
     pub fn move_up_with_selection(&mut self) {
-        self.move_up();
+        let _ = self.move_up();
         // Selection anchor stays in place, cursor position updates automatically
     }
 
     pub fn move_down_with_selection(&mut self) {
-        self.move_down();
+        let _ = self.move_down();
         // Selection anchor stays in place, cursor position updates automatically
     }
 

canvas/src/lib.rs

@@ -35,7 +35,10 @@ pub use canvas::actions::{CanvasAction, ActionResult};
 pub use validation::{
     ValidationConfig, ValidationResult, ValidationError,
     CharacterLimits, ValidationConfigBuilder, ValidationState,
-    ValidationSummary,
+    ValidationSummary, PatternFilters, PositionFilter, PositionRange, CharacterFilter, 
+    DisplayMask,  // Simple display mask instead of complex ReservedCharacters
+    // Feature 4: custom formatting exports
+    CustomFormatter, FormattingResult, PositionMapper, DefaultPositionMapper,
 };
 
 // Theming and GUI

canvas/src/validation/config.rs

@@ -1,32 +1,220 @@
 // src/validation/config.rs
 //! Validation configuration types and builders
 
-use crate::validation::CharacterLimits;
-use serde::{Deserialize, Serialize};
+use crate::validation::{CharacterLimits, PatternFilters, DisplayMask};
+#[cfg(feature = "validation")]
+use crate::validation::{CustomFormatter, FormattingResult, PositionMapper};
+use std::sync::Arc;
 
 /// Main validation configuration for a field
-#[derive(Debug, Clone, Default, Serialize, Deserialize)]
+#[derive(Clone, Default)]
 pub struct ValidationConfig {
     /// Character limit configuration
     pub character_limits: Option<CharacterLimits>,
-    
-    /// Future: Predefined patterns
-    #[serde(skip)]
-    pub patterns: Option<()>, // Placeholder for future implementation
-    
-    /// Future: Reserved characters
-    #[serde(skip)]
-    pub reserved_chars: Option<()>, // Placeholder for future implementation
-    
-    /// Future: Custom formatting
-    #[serde(skip)]
-    pub custom_formatting: Option<()>, // Placeholder for future implementation
-    
+
+    /// Pattern filtering configuration
+    pub pattern_filters: Option<PatternFilters>,
+
+    /// User-defined display mask for visual formatting
+    pub display_mask: Option<DisplayMask>,
+
+    /// Optional: user-provided custom formatter (feature 4)
+    #[cfg(feature = "validation")]
+    pub custom_formatter: Option<Arc<dyn CustomFormatter + Send + Sync>>,
+
+    /// Enable external validation indicator UI (feature 5)
+    pub external_validation_enabled: bool,
+
     /// Future: External validation
-    #[serde(skip)]
     pub external_validation: Option<()>, // Placeholder for future implementation
 }
 
+/// Manual Debug to avoid requiring Debug on dyn CustomFormatter
+impl std::fmt::Debug for ValidationConfig {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let mut ds = f.debug_struct("ValidationConfig");
+        ds.field("character_limits", &self.character_limits)
+          .field("pattern_filters", &self.pattern_filters)
+          .field("display_mask", &self.display_mask)
+          // Do not print the formatter itself to avoid requiring Debug
+          .field(
+              "custom_formatter",
+              &{
+                  #[cfg(feature = "validation")]
+                  {
+                      if self.custom_formatter.is_some() { &"Some(<CustomFormatter>)" } else { &"None" }
+                  }
+                  #[cfg(not(feature = "validation"))]
+                  {
+                      &"N/A"
+                  }
+              },
+          )
+          .field("external_validation_enabled", &self.external_validation_enabled)
+          .field("external_validation", &self.external_validation)
+          .finish()
+    }
+}
+
+// ✅ FIXED: Move function from struct definition to impl block
+impl ValidationConfig {
+    /// If a custom formatter is configured, run it and return the formatted text,
+    /// the position mapper and an optional warning message.
+    ///
+    /// Returns None when no custom formatter is configured.
+    #[cfg(feature = "validation")]
+    pub fn run_custom_formatter(
+        &self,
+        raw: &str,
+    ) -> Option<(String, Arc<dyn PositionMapper>, Option<String>)> {
+        let formatter = self.custom_formatter.as_ref()?;
+        match formatter.format(raw) {
+            FormattingResult::Success { formatted, mapper } => {
+                Some((formatted, mapper, None))
+            }
+            FormattingResult::Warning { formatted, message, mapper } => {
+                Some((formatted, mapper, Some(message)))
+            }
+            FormattingResult::Error { .. } => None, // Fall back to raw display
+        }
+    }
+
+    /// Create a new empty validation configuration
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    /// Create a configuration with just character limits
+    pub fn with_max_length(max_length: usize) -> Self {
+        ValidationConfigBuilder::new()
+            .with_max_length(max_length)
+            .build()
+    }
+
+    /// Create a configuration with pattern filters
+    pub fn with_patterns(patterns: PatternFilters) -> Self {
+        ValidationConfigBuilder::new()
+            .with_pattern_filters(patterns)
+            .build()
+    }
+
+    /// Create a configuration with user-defined display mask
+    ///
+    /// # Examples
+    /// ```
+    /// use canvas::{ValidationConfig, DisplayMask};
+    ///
+    /// let phone_mask = DisplayMask::new("(###) ###-####", '#');
+    /// let config = ValidationConfig::with_mask(phone_mask);
+    /// ```
+    pub fn with_mask(mask: DisplayMask) -> Self {
+        ValidationConfigBuilder::new()
+            .with_display_mask(mask)
+            .build()
+    }
+
+    /// Validate a character insertion at a specific position (raw text space).
+    ///
+    /// Note: Display masks are visual-only and do not participate in validation.
+    /// Editor logic is responsible for skipping mask separator positions; here we
+    /// only validate the raw insertion against limits and patterns.
+    pub fn validate_char_insertion(
+        &self,
+        current_text: &str,
+        position: usize,
+        character: char,
+    ) -> ValidationResult {
+        // Character limits validation
+        if let Some(ref limits) = self.character_limits {
+            // ✅ FIXED: Explicit return type annotation
+            if let Some(result) = limits.validate_insertion(current_text, position, character) {
+                if !result.is_acceptable() {
+                    return result;
+                }
+            }
+        }
+
+        // Pattern filters validation
+        if let Some(ref patterns) = self.pattern_filters {
+            // ✅ FIXED: Explicit error handling
+            if let Err(message) = patterns.validate_char_at_position(position, character) {
+                return ValidationResult::error(message);
+            }
+        }
+
+        // Future: Add other validation types here
+
+        ValidationResult::Valid
+    }
+
+    /// Validate the current text content (raw text space)
+    pub fn validate_content(&self, text: &str) -> ValidationResult {
+        // Character limits validation
+        if let Some(ref limits) = self.character_limits {
+            // ✅ FIXED: Explicit return type annotation
+            if let Some(result) = limits.validate_content(text) {
+                if !result.is_acceptable() {
+                    return result;
+                }
+            }
+        }
+
+        // Pattern filters validation
+        if let Some(ref patterns) = self.pattern_filters {
+            // ✅ FIXED: Explicit error handling
+            if let Err(message) = patterns.validate_text(text) {
+                return ValidationResult::error(message);
+            }
+        }
+
+        // Future: Add other validation types here
+
+        ValidationResult::Valid
+    }
+
+    /// Check if any validation rules are configured
+    pub fn has_validation(&self) -> bool {
+        self.character_limits.is_some()
+            || self.pattern_filters.is_some()
+            || self.display_mask.is_some()
+            || {
+                #[cfg(feature = "validation")]
+                { self.custom_formatter.is_some() }
+                #[cfg(not(feature = "validation"))]
+                { false }
+            }
+    }
+
+    pub fn allows_field_switch(&self, text: &str) -> bool {
+        // Character limits validation
+        if let Some(ref limits) = self.character_limits {
+            // ✅ FIXED: Direct boolean return
+            if !limits.allows_field_switch(text) {
+                return false;
+            }
+        }
+
+        // Future: Add other validation types here
+
+        true
+    }
+
+    /// Get reason why field switching is blocked (if any)
+    pub fn field_switch_block_reason(&self, text: &str) -> Option<String> {
+        // Character limits validation
+        if let Some(ref limits) = self.character_limits {
+            // ✅ FIXED: Direct option return
+            if let Some(reason) = limits.field_switch_block_reason(text) {
+                return Some(reason);
+            }
+        }
+
+        // Future: Add other validation types here
+
+        None
+    }
+}
+
 /// Builder for creating validation configurations
 #[derive(Debug, Default)]
 pub struct ValidationConfigBuilder {
@@ -38,19 +226,75 @@ impl ValidationConfigBuilder {
     pub fn new() -> Self {
         Self::default()
     }
-    
+
     /// Set character limits for the field
     pub fn with_character_limits(mut self, limits: CharacterLimits) -> Self {
         self.config.character_limits = Some(limits);
         self
     }
-    
+
+    /// Set pattern filters for the field
+    pub fn with_pattern_filters(mut self, filters: PatternFilters) -> Self {
+        self.config.pattern_filters = Some(filters);
+        self
+    }
+
+    /// Set user-defined display mask for visual formatting
+    ///
+    /// # Examples
+    /// ```
+    /// use canvas::{ValidationConfigBuilder, DisplayMask};
+    ///
+    /// // Phone number with dynamic formatting
+    /// let phone_mask = DisplayMask::new("(###) ###-####", '#');
+    /// let config = ValidationConfigBuilder::new()
+    ///     .with_display_mask(phone_mask)
+    ///     .build();
+    ///
+    /// // Date with template formatting
+    /// let date_mask = DisplayMask::new("##/##/####", '#')
+    ///     .with_template('_');
+    /// let config = ValidationConfigBuilder::new()
+    ///     .with_display_mask(date_mask)
+    ///     .build();
+    ///
+    /// // Custom business format
+    /// let employee_id = DisplayMask::new("EMP-####-##", '#')
+    ///     .with_template('•');
+    /// let config = ValidationConfigBuilder::new()
+    ///     .with_display_mask(employee_id)
+    ///     .with_max_length(6)  // Only store the 6 digits
+    ///     .build();
+    /// ```
+    pub fn with_display_mask(mut self, mask: DisplayMask) -> Self {
+        self.config.display_mask = Some(mask);
+        self
+    }
+
+    /// Set optional custom formatter (feature 4)
+    #[cfg(feature = "validation")]
+    pub fn with_custom_formatter<F>(mut self, formatter: Arc<F>) -> Self
+    where
+        F: CustomFormatter + Send + Sync + 'static,
+    {
+        self.config.custom_formatter = Some(formatter);
+        // When custom formatter is present, it takes precedence over display mask.
+        self.config.display_mask = None;
+        self
+    }
+
     /// Set maximum number of characters (convenience method)
     pub fn with_max_length(mut self, max_length: usize) -> Self {
         self.config.character_limits = Some(CharacterLimits::new(max_length));
         self
     }
-    
+
+    /// Enable or disable external validation indicator UI (feature 5)
+    pub fn with_external_validation_enabled(mut self, enabled: bool) -> Self {
+        self.config.external_validation_enabled = enabled;
+        self
+    }
+
     /// Build the final validation configuration
     pub fn build(self) -> ValidationConfig {
         self.config
@@ -62,10 +306,10 @@ impl ValidationConfigBuilder {
 pub enum ValidationResult {
     /// Validation passed
     Valid,
-    
+
     /// Validation failed with warning (input still accepted)
     Warning { message: String },
-    
+
     /// Validation failed with error (input rejected)
     Error { message: String },
 }
@@ -75,12 +319,12 @@ impl ValidationResult {
     pub fn is_acceptable(&self) -> bool {
         matches!(self, ValidationResult::Valid | ValidationResult::Warning { .. })
     }
-    
+
     /// Check if the validation result is an error
     pub fn is_error(&self) -> bool {
         matches!(self, ValidationResult::Error { .. })
     }
-    
+
     /// Get the message if there is one
     pub fn message(&self) -> Option<&str> {
         match self {
@@ -89,147 +333,115 @@ impl ValidationResult {
             ValidationResult::Error { message } => Some(message),
         }
     }
-    
+
     /// Create a warning result
     pub fn warning(message: impl Into<String>) -> Self {
         ValidationResult::Warning { message: message.into() }
     }
-    
+
     /// Create an error result
     pub fn error(message: impl Into<String>) -> Self {
         ValidationResult::Error { message: message.into() }
     }
 }
 
-impl ValidationConfig {
-    /// Create a new empty validation configuration
-    pub fn new() -> Self {
-        Self::default()
-    }
-    
-    /// Create a configuration with just character limits
-    pub fn with_max_length(max_length: usize) -> Self {
-        ValidationConfigBuilder::new()
-            .with_max_length(max_length)
-            .build()
-    }
-    
-    /// Validate a character insertion at a specific position
-    pub fn validate_char_insertion(
-        &self,
-        current_text: &str,
-        position: usize,
-        character: char,
-    ) -> ValidationResult {
-        // Character limits validation
-        if let Some(ref limits) = self.character_limits {
-            if let Some(result) = limits.validate_insertion(current_text, position, character) {
-                if !result.is_acceptable() {
-                    return result;
-                }
-            }
-        }
-        
-        // Future: Add other validation types here
-        
-        ValidationResult::Valid
-    }
-    
-    /// Validate the current text content
-    pub fn validate_content(&self, text: &str) -> ValidationResult {
-        // Character limits validation
-        if let Some(ref limits) = self.character_limits {
-            if let Some(result) = limits.validate_content(text) {
-                if !result.is_acceptable() {
-                    return result;
-                }
-            }
-        }
-        
-        // Future: Add other validation types here
-        
-        ValidationResult::Valid
-    }
-    
-    /// Check if any validation rules are configured
-    pub fn has_validation(&self) -> bool {
-        self.character_limits.is_some()
-        // || self.patterns.is_some()
-        // || self.reserved_chars.is_some()
-        // || self.custom_formatting.is_some()
-        // || self.external_validation.is_some()
-    }
-    pub fn allows_field_switch(&self, text: &str) -> bool {
-        // Character limits validation
-        if let Some(ref limits) = self.character_limits {
-            if !limits.allows_field_switch(text) {
-                return false;
-            }
-        }
-        
-        // Future: Add other validation types here
-        
-        true
-    }
-    
-    /// Get reason why field switching is blocked (if any)
-    pub fn field_switch_block_reason(&self, text: &str) -> Option<String> {
-        // Character limits validation
-        if let Some(ref limits) = self.character_limits {
-            if let Some(reason) = limits.field_switch_block_reason(text) {
-                return Some(reason);
-            }
-        }
-        
-        // Future: Add other validation types here
-        
-        None
-    }
-}
-
 #[cfg(test)]
 mod tests {
     use super::*;
 
+    #[test]
+    fn test_config_with_user_defined_mask() {
+        // User creates their own phone mask
+        let phone_mask = DisplayMask::new("(###) ###-####", '#');
+        let config = ValidationConfig::with_mask(phone_mask);
+
+        // has_validation should be true because mask is configured
+        assert!(config.has_validation());
+
+        // Display mask is visual only; validation still focuses on raw content
+        let result = config.validate_char_insertion("123", 3, '4');
+        assert!(result.is_acceptable());
+
+        // Content validation unaffected by mask
+        let result = config.validate_content("1234567890");
+        assert!(result.is_acceptable());
+    }
+
     #[test]
     fn test_validation_config_builder() {
         let config = ValidationConfigBuilder::new()
             .with_max_length(10)
             .build();
-        
+
         assert!(config.character_limits.is_some());
         assert_eq!(config.character_limits.unwrap().max_length(), Some(10));
     }
-    
+
+    #[test]
+    fn test_config_builder_with_user_mask() {
+        // User defines custom format
+        let custom_mask = DisplayMask::new("##-##-##", '#').with_template('_');
+        let config = ValidationConfigBuilder::new()
+            .with_display_mask(custom_mask)
+            .with_max_length(6)
+            .build();
+
+        assert!(config.has_validation());
+        assert!(config.character_limits.is_some());
+        assert!(config.display_mask.is_some());
+    }
+
     #[test]
     fn test_validation_result() {
         let valid = ValidationResult::Valid;
         assert!(valid.is_acceptable());
         assert!(!valid.is_error());
         assert_eq!(valid.message(), None);
-        
+
         let warning = ValidationResult::warning("Too long");
         assert!(warning.is_acceptable());
         assert!(!warning.is_error());
         assert_eq!(warning.message(), Some("Too long"));
-        
+
         let error = ValidationResult::error("Invalid");
         assert!(!error.is_acceptable());
         assert!(error.is_error());
         assert_eq!(error.message(), Some("Invalid"));
     }
-    
+
     #[test]
     fn test_config_with_max_length() {
         let config = ValidationConfig::with_max_length(5);
         assert!(config.has_validation());
-        
+
         // Test valid insertion
         let result = config.validate_char_insertion("test", 4, 'x');
         assert!(result.is_acceptable());
-        
+
         // Test invalid insertion (would exceed limit)
         let result = config.validate_char_insertion("tests", 5, 'x');
         assert!(!result.is_acceptable());
     }
+
+    #[test]
+    fn test_config_with_patterns() {
+        use crate::validation::{PatternFilters, PositionFilter, PositionRange, CharacterFilter};
+
+        let patterns = PatternFilters::new()
+            .add_filter(PositionFilter::new(
+                PositionRange::Range(0, 1),
+                CharacterFilter::Alphabetic,
+            ));
+
+        let config = ValidationConfig::with_patterns(patterns);
+        assert!(config.has_validation());
+
+        // Test valid pattern insertion
+        let result = config.validate_char_insertion("", 0, 'A');
+        assert!(result.is_acceptable());
+
+        // Test invalid pattern insertion
+        let result = config.validate_char_insertion("", 0, '1');
+        assert!(!result.is_acceptable());
+    }
 }

canvas/src/validation/formatting.rs

@@ -0,0 +1,217 @@
+/* canvas/src/validation/formatting.rs
+   Add new formatting module with CustomFormatter, PositionMapper, DefaultPositionMapper, and FormattingResult
+*/
+use std::sync::Arc;
+
+/// Bidirectional mapping between raw input positions and formatted display positions.
+///
+/// The library uses this to keep cursor/selection behavior intuitive when the UI
+/// shows a formatted transformation (e.g., "01001" -> "010 01") while the editor
+/// still stores raw text.
+pub trait PositionMapper: Send + Sync {
+    /// Map a raw cursor position to a formatted cursor position.
+    ///
+    /// raw_pos is an index into the raw text (0..=raw.len() in char positions).
+    /// Implementations should return a position within 0..=formatted.len() (in char positions).
+    fn raw_to_formatted(&self, raw: &str, formatted: &str, raw_pos: usize) -> usize;
+
+    /// Map a formatted cursor position to a raw cursor position.
+    ///
+    /// formatted_pos is an index into the formatted text (0..=formatted.len()).
+    /// Implementations should return a position within 0..=raw.len() (in char positions).
+    fn formatted_to_raw(&self, raw: &str, formatted: &str, formatted_pos: usize) -> usize;
+}
+
+/// A reasonable default mapper that works for "insert separators" style formatting,
+/// such as grouping digits or adding dashes/spaces.
+///
+/// Heuristic:
+/// - Treat letters and digits (is_alphanumeric) in the formatted string as user-entered characters
+///   corresponding to raw characters, in order.
+/// - Treat any non-alphanumeric characters as purely visual separators.
+/// - Raw positions are mapped by counting alphanumeric characters in the formatted string.
+/// - If the formatted contains fewer alphanumeric characters than the raw (shouldn't happen
+///   for plain grouping), we cap at the end of the formatted string.
+#[derive(Clone, Default)]
+pub struct DefaultPositionMapper;
+
+impl PositionMapper for DefaultPositionMapper {
+    fn raw_to_formatted(&self, raw: &str, formatted: &str, raw_pos: usize) -> usize {
+        // Convert to char indices for correctness in presence of UTF-8
+        let raw_len = raw.chars().count();
+        let clamped_raw_pos = raw_pos.min(raw_len);
+
+        // Count alphanumerics in formatted, find the index where we've seen `clamped_raw_pos` of them.
+        let mut seen_user_chars = 0usize;
+        for (idx, ch) in formatted.char_indices() {
+            if ch.is_alphanumeric() {
+                if seen_user_chars == clamped_raw_pos {
+                    // Cursor is positioned before this user character in the formatted view
+                    return idx;
+                }
+                seen_user_chars += 1;
+            }
+        }
+
+        // If we consumed all alphanumeric chars and still haven't reached clamped_raw_pos,
+        // place cursor at the end of the formatted string.
+        formatted.len()
+    }
+
+    fn formatted_to_raw(&self, raw: &str, formatted: &str, formatted_pos: usize) -> usize {
+        let clamped_fmt_pos = formatted_pos.min(formatted.len());
+
+        // Count alphanumerics in formatted up to formatted_pos.
+        let mut seen_user_chars = 0usize;
+        for (idx, ch) in formatted.char_indices() {
+            if idx >= clamped_fmt_pos {
+                break;
+            }
+            if ch.is_alphanumeric() {
+                seen_user_chars += 1;
+            }
+        }
+
+        // Map to raw position by clamping to raw char count
+        let raw_len = raw.chars().count();
+        seen_user_chars.min(raw_len)
+    }
+}
+
+/// Result of invoking a custom formatter on the raw input.
+///
+/// Success variants carry the formatted string and a position mapper to translate
+/// between raw and formatted cursor positions. If you don't provide a custom mapper,
+/// the library will fall back to DefaultPositionMapper.
+pub enum FormattingResult {
+    /// Successfully produced a formatted display value and a position mapper.
+    Success {
+        formatted: String,
+        /// Mapper to convert cursor positions between raw and formatted representations.
+        mapper: Arc<dyn PositionMapper>,
+    },
+    /// Successfully produced a formatted value, but with a non-fatal warning message
+    /// that can be shown in the UI (e.g., "incomplete value").
+    Warning {
+        formatted: String,
+        message: String,
+        mapper: Arc<dyn PositionMapper>,
+    },
+    /// Failed to produce a formatted display. The library will typically fall back to raw.
+    Error {
+        message: String,
+    },
+}
+
+impl FormattingResult {
+    /// Convenience to create a success result using the default mapper.
+    pub fn success(formatted: impl Into<String>) -> Self {
+        FormattingResult::Success {
+            formatted: formatted.into(),
+            mapper: Arc::new(DefaultPositionMapper::default()),
+        }
+    }
+
+    /// Convenience to create a warning result using the default mapper.
+    pub fn warning(formatted: impl Into<String>, message: impl Into<String>) -> Self {
+        FormattingResult::Warning {
+            formatted: formatted.into(),
+            message: message.into(),
+            mapper: Arc::new(DefaultPositionMapper::default()),
+        }
+    }
+
+    /// Convenience to create a success result with a custom mapper.
+    pub fn success_with_mapper(
+        formatted: impl Into<String>,
+        mapper: Arc<dyn PositionMapper>,
+    ) -> Self {
+        FormattingResult::Success {
+            formatted: formatted.into(),
+            mapper,
+        }
+    }
+
+    /// Convenience to create a warning result with a custom mapper.
+    pub fn warning_with_mapper(
+        formatted: impl Into<String>,
+        message: impl Into<String>,
+        mapper: Arc<dyn PositionMapper>,
+    ) -> Self {
+        FormattingResult::Warning {
+            formatted: formatted.into(),
+            message: message.into(),
+            mapper,
+        }
+    }
+
+    /// Convenience to create an error result.
+    pub fn error(message: impl Into<String>) -> Self {
+        FormattingResult::Error {
+            message: message.into(),
+        }
+    }
+}
+
+/// A user-implemented formatter that turns raw input into a formatted display string,
+/// optionally providing a custom cursor position mapper.
+///
+/// Notes:
+/// - The library will keep raw input authoritative for editing and validation.
+/// - The formatted value is only used for display.
+/// - If formatting fails, return Error; the library will show the raw value.
+/// - For common grouping (spaces/dashes), you can return Success/Warning and rely
+///   on DefaultPositionMapper, or provide your own mapper for advanced cases
+///   (reordering, compression, locale-specific rules, etc.).
+pub trait CustomFormatter: Send + Sync {
+    fn format(&self, raw: &str) -> FormattingResult;
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    struct GroupEvery3;
+    impl CustomFormatter for GroupEvery3 {
+        fn format(&self, raw: &str) -> FormattingResult {
+            let mut out = String::new();
+            for (i, ch) in raw.chars().enumerate() {
+                if i > 0 && i % 3 == 0 {
+                    out.push(' ');
+                }
+                out.push(ch);
+            }
+            FormattingResult::success(out)
+        }
+    }
+
+    #[test]
+    fn default_mapper_roundtrip_basic() {
+        let mapper = DefaultPositionMapper::default();
+        let raw = "01001";
+        let formatted = "010 01";
+
+        // raw_to_formatted monotonicity and bounds
+        for rp in 0..=raw.chars().count() {
+            let fp = mapper.raw_to_formatted(raw, formatted, rp);
+            assert!(fp <= formatted.len());
+        }
+
+        // formatted_to_raw bounds
+        for fp in 0..=formatted.len() {
+            let rp = mapper.formatted_to_raw(raw, formatted, fp);
+            assert!(rp <= raw.chars().count());
+        }
+    }
+
+    #[test]
+    fn formatter_groups_every_3() {
+        let f = GroupEvery3;
+        match f.format("1234567") {
+            FormattingResult::Success { formatted, .. } => {
+                assert_eq!(formatted, "123 456 7");
+            }
+            _ => panic!("expected success"),
+        }
+    }
+}

canvas/src/validation/mask.rs

@@ -0,0 +1,333 @@
+// src/validation/mask.rs
+//! Pure display mask system - user-defined patterns only
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum MaskDisplayMode {
+    /// Only show separators as user types
+    /// Example: "" → "", "123" → "123", "12345" → "(123) 45"
+    Dynamic,
+    
+    /// Show full template with placeholders from start
+    /// Example: "" → "(___) ___-____", "123" → "(123) ___-____"
+    Template { 
+        /// Character to use as placeholder for empty input positions
+        placeholder: char 
+    },
+}
+
+impl Default for MaskDisplayMode {
+    fn default() -> Self {
+        MaskDisplayMode::Dynamic
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct DisplayMask {
+    /// Mask pattern like "##-##-####" where # = input position, others are visual separators
+    pattern: String,
+    /// Character used to represent input positions (usually '#')
+    input_char: char,
+    /// How to display the mask (dynamic vs template)
+    display_mode: MaskDisplayMode,
+}
+
+impl DisplayMask {
+    /// Create a new display mask with dynamic mode (current behavior)
+    /// 
+    /// # Arguments
+    /// * `pattern` - The mask pattern (e.g., "##-##-####", "(###) ###-####")  
+    /// * `input_char` - Character representing input positions (usually '#')
+    /// 
+    /// # Examples
+    /// ```
+    /// // Phone number format
+    /// let phone_mask = DisplayMask::new("(###) ###-####", '#');
+    /// 
+    /// // Date format  
+    /// let date_mask = DisplayMask::new("##/##/####", '#');
+    /// 
+    /// // Custom business format
+    /// let employee_id = DisplayMask::new("EMP-####-##", '#');
+    /// ```
+    pub fn new(pattern: impl Into<String>, input_char: char) -> Self {
+        Self {
+            pattern: pattern.into(),
+            input_char,
+            display_mode: MaskDisplayMode::Dynamic,
+        }
+    }
+
+    /// Set the display mode for this mask
+    /// 
+    /// # Examples
+    /// ```
+    /// let dynamic_mask = DisplayMask::new("##-##", '#')
+    ///     .with_mode(MaskDisplayMode::Dynamic);
+    ///     
+    /// let template_mask = DisplayMask::new("##-##", '#')
+    ///     .with_mode(MaskDisplayMode::Template { placeholder: '_' });
+    /// ```
+    pub fn with_mode(mut self, mode: MaskDisplayMode) -> Self {
+        self.display_mode = mode;
+        self
+    }
+
+    /// Set template mode with custom placeholder
+    /// 
+    /// # Examples
+    /// ```
+    /// let phone_template = DisplayMask::new("(###) ###-####", '#')
+    ///     .with_template('_');  // Shows "(___) ___-____" when empty
+    ///     
+    /// let date_dots = DisplayMask::new("##/##/####", '#')
+    ///     .with_template('•');  // Shows "••/••/••••" when empty
+    /// ```
+    pub fn with_template(self, placeholder: char) -> Self {
+        self.with_mode(MaskDisplayMode::Template { placeholder })
+    }
+
+    /// Apply mask to raw input, showing visual separators and handling display mode
+    pub fn apply_to_display(&self, raw_input: &str) -> String {
+        match &self.display_mode {
+            MaskDisplayMode::Dynamic => self.apply_dynamic(raw_input),
+            MaskDisplayMode::Template { placeholder } => self.apply_template(raw_input, *placeholder),
+        }
+    }
+
+    /// Dynamic mode - only show separators as user types
+    fn apply_dynamic(&self, raw_input: &str) -> String {
+        let mut result = String::new();
+        let mut raw_chars = raw_input.chars();
+
+        for pattern_char in self.pattern.chars() {
+            if pattern_char == self.input_char {
+                // Input position - take from raw input
+                if let Some(input_char) = raw_chars.next() {
+                    result.push(input_char);
+                } else {
+                    // No more input - stop here in dynamic mode
+                    break;
+                }
+            } else {
+                // Visual separator - always show
+                result.push(pattern_char);
+            }
+        }
+
+        // Append any remaining raw characters that don't fit the pattern
+        for remaining_char in raw_chars {
+            result.push(remaining_char);
+        }
+
+        result
+    }
+
+    /// Template mode - show full pattern with placeholders
+    fn apply_template(&self, raw_input: &str, placeholder: char) -> String {
+        let mut result = String::new();
+        let mut raw_chars = raw_input.chars().peekable();
+
+        for pattern_char in self.pattern.chars() {
+            if pattern_char == self.input_char {
+                // Input position - take from raw input or use placeholder
+                if let Some(input_char) = raw_chars.next() {
+                    result.push(input_char);
+                } else {
+                    // No more input - use placeholder to show template
+                    result.push(placeholder);
+                }
+            } else {
+                // Visual separator - always show in template mode
+                result.push(pattern_char);
+            }
+        }
+
+        // In template mode, we don't append extra characters beyond the pattern
+        // This keeps the template consistent
+        result
+    }
+
+    /// Check if a display position should accept cursor/input
+    pub fn is_input_position(&self, display_position: usize) -> bool {
+        self.pattern.chars()
+            .nth(display_position)
+            .map(|c| c == self.input_char)
+            .unwrap_or(true) // Beyond pattern = accept input
+    }
+
+    /// Map display position to raw position
+    pub fn display_pos_to_raw_pos(&self, display_pos: usize) -> usize {
+        let mut raw_pos = 0;
+
+        for (i, pattern_char) in self.pattern.chars().enumerate() {
+            if i >= display_pos {
+                break;
+            }
+            if pattern_char == self.input_char {
+                raw_pos += 1;
+            }
+        }
+
+        raw_pos
+    }
+
+    /// Map raw position to display position
+    pub fn raw_pos_to_display_pos(&self, raw_pos: usize) -> usize {
+        let mut input_positions_seen = 0;
+
+        for (display_pos, pattern_char) in self.pattern.chars().enumerate() {
+            if pattern_char == self.input_char {
+                if input_positions_seen == raw_pos {
+                    return display_pos;
+                }
+                input_positions_seen += 1;
+            }
+        }
+
+        // Beyond pattern, return position after pattern
+        self.pattern.len() + (raw_pos - input_positions_seen)
+    }
+
+    /// Find next input position at or after the given display position
+    pub fn next_input_position(&self, display_pos: usize) -> usize {
+        for (i, pattern_char) in self.pattern.chars().enumerate().skip(display_pos) {
+            if pattern_char == self.input_char {
+                return i;
+            }
+        }
+        // Beyond pattern = all positions are input positions
+        display_pos.max(self.pattern.len())
+    }
+
+    /// Find previous input position at or before the given display position
+    pub fn prev_input_position(&self, display_pos: usize) -> Option<usize> {
+        // Collect pattern chars with indices first, then search backwards
+        let pattern_chars: Vec<(usize, char)> = self.pattern.chars().enumerate().collect();
+        
+        // Search backwards from display_pos
+        for &(i, pattern_char) in pattern_chars.iter().rev() {
+            if i <= display_pos && pattern_char == self.input_char {
+                return Some(i);
+            }
+        }
+        None
+    }
+
+    /// Get the display mode
+    pub fn display_mode(&self) -> &MaskDisplayMode {
+        &self.display_mode
+    }
+
+    /// Check if this mask uses template mode
+    pub fn is_template_mode(&self) -> bool {
+        matches!(self.display_mode, MaskDisplayMode::Template { .. })
+    }
+
+    /// Get the pattern string
+    pub fn pattern(&self) -> &str {
+        &self.pattern
+    }
+
+    /// Get the position of the first input character in the pattern
+    pub fn first_input_position(&self) -> usize {
+        for (pos, ch) in self.pattern.chars().enumerate() {
+            if ch == self.input_char {
+                return pos;
+            }
+        }
+        0
+    }
+}
+
+impl Default for DisplayMask {
+    fn default() -> Self {
+        Self::new("", '#')
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_user_defined_phone_mask() {
+        // User creates their own phone mask
+        let dynamic = DisplayMask::new("(###) ###-####", '#');
+        let template = DisplayMask::new("(###) ###-####", '#').with_template('_');
+        
+        // Dynamic mode
+        assert_eq!(dynamic.apply_to_display(""), "");
+        assert_eq!(dynamic.apply_to_display("1234567890"), "(123) 456-7890");
+        
+        // Template mode
+        assert_eq!(template.apply_to_display(""), "(___) ___-____");
+        assert_eq!(template.apply_to_display("123"), "(123) ___-____");
+    }
+
+    #[test]
+    fn test_user_defined_date_mask() {
+        // User creates their own date formats
+        let us_date = DisplayMask::new("##/##/####", '#');
+        let eu_date = DisplayMask::new("##.##.####", '#');
+        let iso_date = DisplayMask::new("####-##-##", '#');
+        
+        assert_eq!(us_date.apply_to_display("12252024"), "12/25/2024");
+        assert_eq!(eu_date.apply_to_display("25122024"), "25.12.2024");
+        assert_eq!(iso_date.apply_to_display("20241225"), "2024-12-25");
+    }
+
+    #[test]
+    fn test_user_defined_business_formats() {
+        // User creates custom business formats
+        let employee_id = DisplayMask::new("EMP-####-##", '#');
+        let product_code = DisplayMask::new("###-###-###", '#');
+        let invoice = DisplayMask::new("INV####/##", '#');
+        
+        assert_eq!(employee_id.apply_to_display("123456"), "EMP-1234-56");
+        assert_eq!(product_code.apply_to_display("123456789"), "123-456-789");
+        assert_eq!(invoice.apply_to_display("123456"), "INV1234/56");
+    }
+
+    #[test]
+    fn test_custom_input_characters() {
+        // User can define their own input character
+        let mask_with_x = DisplayMask::new("XXX-XX-XXXX", 'X');
+        let mask_with_hash = DisplayMask::new("###-##-####", '#');
+        let mask_with_n = DisplayMask::new("NNN-NN-NNNN", 'N');
+        
+        assert_eq!(mask_with_x.apply_to_display("123456789"), "123-45-6789");
+        assert_eq!(mask_with_hash.apply_to_display("123456789"), "123-45-6789");
+        assert_eq!(mask_with_n.apply_to_display("123456789"), "123-45-6789");
+    }
+
+    #[test]
+    fn test_custom_placeholders() {
+        // User can define custom placeholder characters
+        let underscores = DisplayMask::new("##-##", '#').with_template('_');
+        let dots = DisplayMask::new("##-##", '#').with_template('•');
+        let dashes = DisplayMask::new("##-##", '#').with_template('-');
+        
+        assert_eq!(underscores.apply_to_display(""), "__-__");
+        assert_eq!(dots.apply_to_display(""), "••-••");
+        assert_eq!(dashes.apply_to_display(""), "---");  // Note: dashes blend with separator
+    }
+
+    #[test]
+    fn test_position_mapping_user_patterns() {
+        let custom = DisplayMask::new("ABC-###-XYZ", '#');
+        
+        // Position mapping should work correctly with any pattern
+        assert_eq!(custom.raw_pos_to_display_pos(0), 4); // First # at position 4
+        assert_eq!(custom.raw_pos_to_display_pos(1), 5); // Second # at position 5
+        assert_eq!(custom.raw_pos_to_display_pos(2), 6); // Third # at position 6
+        
+        assert_eq!(custom.display_pos_to_raw_pos(4), 0); // Position 4 -> first input
+        assert_eq!(custom.display_pos_to_raw_pos(5), 1); // Position 5 -> second input
+        assert_eq!(custom.display_pos_to_raw_pos(6), 2); // Position 6 -> third input
+        
+        assert!(!custom.is_input_position(0)); // A
+        assert!(!custom.is_input_position(3)); // -
+        assert!(custom.is_input_position(4));  // #
+        assert!(!custom.is_input_position(8)); // Y
+    }
+}

canvas/src/validation/mod.rs

@@ -1,26 +1,40 @@
-//! Validation module for canvas form fields
+// src/validation/mod.rs
 
+// Core validation modules
 pub mod config;
 pub mod limits;
 pub mod state;
+pub mod patterns;
+pub mod mask;  // Simple display mask instead of complex reserved chars
+pub mod formatting; // Custom formatter and position mapping (feature 4)
 
 // Re-export main types
 pub use config::{ValidationConfig, ValidationResult, ValidationConfigBuilder};
 pub use limits::{CharacterLimits, LimitCheckResult};
 pub use state::{ValidationState, ValidationSummary};
+pub use patterns::{PatternFilters, PositionFilter, PositionRange, CharacterFilter};
+pub use mask::DisplayMask;  // Simple mask instead of ReservedCharacters
+pub use formatting::{CustomFormatter, FormattingResult, PositionMapper, DefaultPositionMapper};
+
+/// External validation UI state (Feature 5)
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum ExternalValidationState {
+    NotValidated,
+    Validating,
+    Valid(Option<String>),
+    Invalid { message: String, suggestion: Option<String> },
+    Warning { message: String },
+}
 
 /// Validation error types
 #[derive(Debug, Clone, thiserror::Error)]
 pub enum ValidationError {
-    #[error("Character limit exceeded: {current}/{max}")]
-    CharacterLimitExceeded { current: usize, max: usize },
+    #[error("Character limit exceeded: {message}")]
+    LimitExceeded { message: String },
     
-    #[error("Invalid character '{char}' at position {position}")]
-    InvalidCharacter { char: char, position: usize },
+    #[error("Pattern validation failed: {message}")]
+    PatternFailed { message: String },
     
-    #[error("Validation configuration error: {message}")]
-    ConfigurationError { message: String },
+    #[error("Custom validation failed: {message}")]
+    CustomFailed { message: String },
 }
-
-/// Result type for validation operations
-pub type Result<T> = std::result::Result<T, ValidationError>;

canvas/src/validation/patterns.rs

@@ -0,0 +1,326 @@
+// src/validation/patterns.rs
+//! Position-based pattern filtering for validation
+
+use serde::{Deserialize, Serialize};
+use std::sync::Arc;
+
+/// A filter that applies to specific character positions in a field
+#[derive(Debug, Clone)]
+pub struct PositionFilter {
+    /// Which positions this filter applies to
+    pub positions: PositionRange,
+    /// What type of character filter to apply
+    pub filter: CharacterFilter,
+}
+
+/// Defines which character positions a filter applies to
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub enum PositionRange {
+    /// Single position (e.g., position 3 only)
+    Single(usize),
+    /// Range of positions (e.g., positions 0-2, inclusive)
+    Range(usize, usize),
+    /// From position onwards (e.g., position 4 and beyond)
+    From(usize),
+    /// Multiple specific positions (e.g., positions 0, 2, 5)
+    Multiple(Vec<usize>),
+}
+
+/// Types of character filters that can be applied
+pub enum CharacterFilter {
+    /// Allow only alphabetic characters (a-z, A-Z)
+    Alphabetic,
+    /// Allow only numeric characters (0-9)
+    Numeric,
+    /// Allow alphanumeric characters (a-z, A-Z, 0-9)
+    Alphanumeric,
+    /// Allow only exact character match
+    Exact(char),
+    /// Allow any character from the provided set
+    OneOf(Vec<char>),
+    /// Custom user-defined filter function
+    Custom(Arc<dyn Fn(char) -> bool + Send + Sync>),
+}
+
+// Manual implementations for Debug and Clone
+impl std::fmt::Debug for CharacterFilter {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            CharacterFilter::Alphabetic => write!(f, "Alphabetic"),
+            CharacterFilter::Numeric => write!(f, "Numeric"),
+            CharacterFilter::Alphanumeric => write!(f, "Alphanumeric"),
+            CharacterFilter::Exact(ch) => write!(f, "Exact('{}')", ch),
+            CharacterFilter::OneOf(chars) => write!(f, "OneOf({:?})", chars),
+            CharacterFilter::Custom(_) => write!(f, "Custom(<function>)"),
+        }
+    }
+}
+
+impl Clone for CharacterFilter {
+    fn clone(&self) -> Self {
+        match self {
+            CharacterFilter::Alphabetic => CharacterFilter::Alphabetic,
+            CharacterFilter::Numeric => CharacterFilter::Numeric,
+            CharacterFilter::Alphanumeric => CharacterFilter::Alphanumeric,
+            CharacterFilter::Exact(ch) => CharacterFilter::Exact(*ch),
+            CharacterFilter::OneOf(chars) => CharacterFilter::OneOf(chars.clone()),
+            CharacterFilter::Custom(func) => CharacterFilter::Custom(Arc::clone(func)),
+        }
+    }
+}
+
+impl PositionRange {
+    /// Check if a position is included in this range
+    pub fn contains(&self, position: usize) -> bool {
+        match self {
+            PositionRange::Single(pos) => position == *pos,
+            PositionRange::Range(start, end) => position >= *start && position <= *end,
+            PositionRange::From(start) => position >= *start,
+            PositionRange::Multiple(positions) => positions.contains(&position),
+        }
+    }
+
+    /// Get all positions up to a given length that this range covers
+    pub fn positions_up_to(&self, max_length: usize) -> Vec<usize> {
+        match self {
+            PositionRange::Single(pos) => {
+                if *pos < max_length { vec![*pos] } else { vec![] }
+            },
+            PositionRange::Range(start, end) => {
+                let actual_end = (*end).min(max_length.saturating_sub(1));
+                if *start <= actual_end {
+                    (*start..=actual_end).collect()
+                } else {
+                    vec![]
+                }
+            },
+            PositionRange::From(start) => {
+                if *start < max_length {
+                    (*start..max_length).collect()
+                } else {
+                    vec![]
+                }
+            },
+            PositionRange::Multiple(positions) => {
+                positions.iter()
+                    .filter(|&&pos| pos < max_length)
+                    .copied()
+                    .collect()
+            },
+        }
+    }
+}
+
+impl CharacterFilter {
+    /// Test if a character passes this filter
+    pub fn accepts(&self, ch: char) -> bool {
+        match self {
+            CharacterFilter::Alphabetic => ch.is_alphabetic(),
+            CharacterFilter::Numeric => ch.is_numeric(),
+            CharacterFilter::Alphanumeric => ch.is_alphanumeric(),
+            CharacterFilter::Exact(expected) => ch == *expected,
+            CharacterFilter::OneOf(chars) => chars.contains(&ch),
+            CharacterFilter::Custom(func) => func(ch),
+        }
+    }
+
+    /// Get a human-readable description of this filter
+    pub fn description(&self) -> String {
+        match self {
+            CharacterFilter::Alphabetic => "alphabetic characters (a-z, A-Z)".to_string(),
+            CharacterFilter::Numeric => "numeric characters (0-9)".to_string(),
+            CharacterFilter::Alphanumeric => "alphanumeric characters (a-z, A-Z, 0-9)".to_string(),
+            CharacterFilter::Exact(ch) => format!("exactly '{}'", ch),
+            CharacterFilter::OneOf(chars) => {
+                let char_list: String = chars.iter().collect();
+                format!("one of: {}", char_list)
+            },
+            CharacterFilter::Custom(_) => "custom filter".to_string(),
+        }
+    }
+}
+
+impl PositionFilter {
+    /// Create a new position filter
+    pub fn new(positions: PositionRange, filter: CharacterFilter) -> Self {
+        Self { positions, filter }
+    }
+
+    /// Validate a character at a specific position
+    pub fn validate_position(&self, position: usize, character: char) -> bool {
+        if self.positions.contains(position) {
+            self.filter.accepts(character)
+        } else {
+            true // Position not covered by this filter, allow any character
+        }
+    }
+
+    /// Get error message for invalid character at position
+    pub fn error_message(&self, position: usize, character: char) -> Option<String> {
+        if self.positions.contains(position) && !self.filter.accepts(character) {
+            Some(format!(
+                "Position {} requires {} but got '{}'",
+                position,
+                self.filter.description(),
+                character
+            ))
+        } else {
+            None
+        }
+    }
+}
+
+/// A collection of position filters for a field
+#[derive(Debug, Clone, Default)]
+pub struct PatternFilters {
+    filters: Vec<PositionFilter>,
+}
+
+impl PatternFilters {
+    /// Create empty pattern filters
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    /// Add a position filter
+    pub fn add_filter(mut self, filter: PositionFilter) -> Self {
+        self.filters.push(filter);
+        self
+    }
+
+    /// Add multiple filters
+    pub fn add_filters(mut self, filters: Vec<PositionFilter>) -> Self {
+        self.filters.extend(filters);
+        self
+    }
+
+    /// Validate a character at a specific position against all applicable filters
+    pub fn validate_char_at_position(&self, position: usize, character: char) -> Result<(), String> {
+        for filter in &self.filters {
+            if let Some(error) = filter.error_message(position, character) {
+                return Err(error);
+            }
+        }
+        Ok(())
+    }
+
+    /// Validate entire text against all filters
+    pub fn validate_text(&self, text: &str) -> Result<(), String> {
+        for (position, character) in text.char_indices() {
+            if let Err(error) = self.validate_char_at_position(position, character) {
+                return Err(error);
+            }
+        }
+        Ok(())
+    }
+
+    /// Check if any filters are configured
+    pub fn has_filters(&self) -> bool {
+        !self.filters.is_empty()
+    }
+
+    /// Get all configured filters
+    pub fn filters(&self) -> &[PositionFilter] {
+        &self.filters
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_position_range_contains() {
+        assert!(PositionRange::Single(3).contains(3));
+        assert!(!PositionRange::Single(3).contains(2));
+
+        assert!(PositionRange::Range(1, 4).contains(3));
+        assert!(!PositionRange::Range(1, 4).contains(5));
+
+        assert!(PositionRange::From(2).contains(5));
+        assert!(!PositionRange::From(2).contains(1));
+
+        assert!(PositionRange::Multiple(vec![0, 2, 5]).contains(2));
+        assert!(!PositionRange::Multiple(vec![0, 2, 5]).contains(3));
+    }
+
+    #[test]
+    fn test_position_range_positions_up_to() {
+        assert_eq!(PositionRange::Single(3).positions_up_to(5), vec![3]);
+        assert_eq!(PositionRange::Single(5).positions_up_to(3), vec![]);
+
+        assert_eq!(PositionRange::Range(1, 3).positions_up_to(5), vec![1, 2, 3]);
+        assert_eq!(PositionRange::Range(1, 5).positions_up_to(3), vec![1, 2]);
+
+        assert_eq!(PositionRange::From(2).positions_up_to(5), vec![2, 3, 4]);
+
+        assert_eq!(PositionRange::Multiple(vec![0, 2, 5]).positions_up_to(4), vec![0, 2]);
+    }
+
+    #[test]
+    fn test_character_filter_accepts() {
+        assert!(CharacterFilter::Alphabetic.accepts('a'));
+        assert!(CharacterFilter::Alphabetic.accepts('Z'));
+        assert!(!CharacterFilter::Alphabetic.accepts('1'));
+
+        assert!(CharacterFilter::Numeric.accepts('5'));
+        assert!(!CharacterFilter::Numeric.accepts('a'));
+
+        assert!(CharacterFilter::Alphanumeric.accepts('a'));
+        assert!(CharacterFilter::Alphanumeric.accepts('5'));
+        assert!(!CharacterFilter::Alphanumeric.accepts('-'));
+
+        assert!(CharacterFilter::Exact('x').accepts('x'));
+        assert!(!CharacterFilter::Exact('x').accepts('y'));
+
+        assert!(CharacterFilter::OneOf(vec!['a', 'b', 'c']).accepts('b'));
+        assert!(!CharacterFilter::OneOf(vec!['a', 'b', 'c']).accepts('d'));
+    }
+
+    #[test]
+    fn test_position_filter_validation() {
+        let filter = PositionFilter::new(
+            PositionRange::Range(0, 1),
+            CharacterFilter::Alphabetic,
+        );
+
+        assert!(filter.validate_position(0, 'A'));
+        assert!(filter.validate_position(1, 'b'));
+        assert!(!filter.validate_position(0, '1'));
+        assert!(filter.validate_position(2, '1')); // Position 2 not covered, allow anything
+    }
+
+    #[test]
+    fn test_pattern_filters_validation() {
+        let patterns = PatternFilters::new()
+            .add_filter(PositionFilter::new(
+                PositionRange::Range(0, 1),
+                CharacterFilter::Alphabetic,
+            ))
+            .add_filter(PositionFilter::new(
+                PositionRange::Range(2, 4),
+                CharacterFilter::Numeric,
+            ));
+
+        // Valid pattern: AB123
+        assert!(patterns.validate_text("AB123").is_ok());
+
+        // Invalid: number in alphabetic position
+        assert!(patterns.validate_text("A1123").is_err());
+
+        // Invalid: letter in numeric position
+        assert!(patterns.validate_text("AB1A3").is_err());
+    }
+
+    #[test]
+    fn test_custom_filter() {
+        let pattern = PatternFilters::new()
+            .add_filter(PositionFilter::new(
+                PositionRange::From(0),
+                CharacterFilter::Custom(Arc::new(|c| c.is_lowercase())),
+            ));
+
+        assert!(pattern.validate_text("hello").is_ok());
+        assert!(pattern.validate_text("Hello").is_err()); // Uppercase not allowed
+    }
+}

canvas/src/validation/state.rs

@@ -1,7 +1,7 @@
 // src/validation/state.rs
 //! Validation state management
 
-use crate::validation::{ValidationConfig, ValidationResult};
+use crate::validation::{ValidationConfig, ValidationResult, ExternalValidationState};
 use std::collections::HashMap;
 
 /// Validation state for all fields in a form
@@ -18,6 +18,9 @@ pub struct ValidationState {
     
     /// Global validation enabled/disabled
     enabled: bool,
+
+    /// External validation results per field (Feature 5)
+    external_results: HashMap<usize, ExternalValidationState>,
 }
 
 impl ValidationState {
@@ -28,6 +31,7 @@ impl ValidationState {
             field_results: HashMap::new(),
             validated_fields: std::collections::HashSet::new(),
             enabled: true,
+            external_results: HashMap::new(),
         }
     }
     
@@ -38,6 +42,7 @@ impl ValidationState {
             // Clear all validation results when disabled
             self.field_results.clear();
             self.validated_fields.clear();
+            self.external_results.clear(); // Also clear external results
         }
     }
     
@@ -48,12 +53,13 @@ impl ValidationState {
     
     /// Set validation configuration for a field
     pub fn set_field_config(&mut self, field_index: usize, config: ValidationConfig) {
-        if config.has_validation() {
+        if config.has_validation() || config.external_validation_enabled {
             self.field_configs.insert(field_index, config);
         } else {
             self.field_configs.remove(&field_index);
             self.field_results.remove(&field_index);
             self.validated_fields.remove(&field_index);
+            self.external_results.remove(&field_index);
         }
     }
     
@@ -67,6 +73,30 @@ impl ValidationState {
         self.field_configs.remove(&field_index);
         self.field_results.remove(&field_index);
         self.validated_fields.remove(&field_index);
+        self.external_results.remove(&field_index);
+    }
+    
+    /// Set external validation state for a field (Feature 5)
+    pub fn set_external_validation(&mut self, field_index: usize, state: ExternalValidationState) {
+        self.external_results.insert(field_index, state);
+    }
+    
+    /// Get current external validation state for a field
+    pub fn get_external_validation(&self, field_index: usize) -> ExternalValidationState {
+        self.external_results
+            .get(&field_index)
+            .cloned()
+            .unwrap_or(ExternalValidationState::NotValidated)
+    }
+    
+    /// Clear external validation state for a field
+    pub fn clear_external_validation(&mut self, field_index: usize) {
+        self.external_results.remove(&field_index);
+    }
+    
+    /// Clear all external validation states
+    pub fn clear_all_external_validation(&mut self) {
+        self.external_results.clear();
     }
     
     /// Validate character insertion for a field
@@ -122,6 +152,18 @@ impl ValidationState {
         self.field_results.get(&field_index)
     }
     
+    /// Get formatted display for a field if a custom formatter is configured.
+    /// Returns (formatted_text, position_mapper, optional_warning_message).
+    #[cfg(feature = "validation")]
+    pub fn formatted_for(
+        &self,
+        field_index: usize,
+        raw: &str,
+    ) -> Option<(String, std::sync::Arc<dyn crate::validation::PositionMapper>, Option<String>)> {
+        let config = self.field_configs.get(&field_index)?;
+        config.run_custom_formatter(raw)
+    }
+    
     /// Check if a field has been validated
     pub fn is_field_validated(&self, field_index: usize) -> bool {
         self.validated_fields.contains(&field_index)