How to define TextBox input restrictions?

I've done this in the past with an attached behavior, which can be used like this:

<TextBox b:Masking.Mask="^\p{Lu}*$"/>

The attached behavior code looks like this:

/// <summary>
/// Provides masking behavior for any <see cref="TextBox"/>.
/// </summary>
public static class Masking
{
    private static readonly DependencyPropertyKey _maskExpressionPropertyKey = DependencyProperty.RegisterAttachedReadOnly("MaskExpression",
        typeof(Regex),
        typeof(Masking),
        new FrameworkPropertyMetadata());

    /// <summary>
    /// Identifies the <see cref="Mask"/> dependency property.
    /// </summary>
    public static readonly DependencyProperty MaskProperty = DependencyProperty.RegisterAttached("Mask",
        typeof(string),
        typeof(Masking),
        new FrameworkPropertyMetadata(OnMaskChanged));

    /// <summary>
    /// Identifies the <see cref="MaskExpression"/> dependency property.
    /// </summary>
    public static readonly DependencyProperty MaskExpressionProperty = _maskExpressionPropertyKey.DependencyProperty;

    /// <summary>
    /// Gets the mask for a given <see cref="TextBox"/>.
    /// </summary>
    /// <param name="textBox">
    /// The <see cref="TextBox"/> whose mask is to be retrieved.
    /// </param>
    /// <returns>
    /// The mask, or <see langword="null"/> if no mask has been set.
    /// </returns>
    public static string GetMask(TextBox textBox)
    {
        if (textBox == null)
        {
            throw new ArgumentNullException("textBox");
        }

        return textBox.GetValue(MaskProperty) as string;
    }

    /// <summary>
    /// Sets the mask for a given <see cref="TextBox"/>.
    /// </summary>
    /// <param name="textBox">
    /// The <see cref="TextBox"/> whose mask is to be set.
    /// </param>
    /// <param name="mask">
    /// The mask to set, or <see langword="null"/> to remove any existing mask from <paramref name="textBox"/>.
    /// </param>
    public static void SetMask(TextBox textBox, string mask)
    {
        if (textBox == null)
        {
            throw new ArgumentNullException("textBox");
        }

        textBox.SetValue(MaskProperty, mask);
    }

    /// <summary>
    /// Gets the mask expression for the <see cref="TextBox"/>.
    /// </summary>
    /// <remarks>
    /// This method can be used to retrieve the actual <see cref="Regex"/> instance created as a result of setting the mask on a <see cref="TextBox"/>.
    /// </remarks>
    /// <param name="textBox">
    /// The <see cref="TextBox"/> whose mask expression is to be retrieved.
    /// </param>
    /// <returns>
    /// The mask expression as an instance of <see cref="Regex"/>, or <see langword="null"/> if no mask has been applied to <paramref name="textBox"/>.
    /// </returns>
    public static Regex GetMaskExpression(TextBox textBox)
    {
        if (textBox == null)
        {
            throw new ArgumentNullException("textBox");
        } 

        return textBox.GetValue(MaskExpressionProperty) as Regex;
    }

    private static void SetMaskExpression(TextBox textBox, Regex regex)
    {
        textBox.SetValue(_maskExpressionPropertyKey, regex);
    }

    private static void OnMaskChanged(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs e)
    {
        var textBox = dependencyObject as TextBox;
        var mask = e.NewValue as string;
        textBox.PreviewTextInput -= textBox_PreviewTextInput;
        textBox.PreviewKeyDown -= textBox_PreviewKeyDown;
        DataObject.RemovePastingHandler(textBox, Pasting);

        if (mask == null)
        {
            textBox.ClearValue(MaskProperty);
            textBox.ClearValue(MaskExpressionProperty);
        }
        else
        {
            textBox.SetValue(MaskProperty, mask);
            SetMaskExpression(textBox, new Regex(mask, RegexOptions.Compiled | RegexOptions.IgnorePatternWhitespace));
            textBox.PreviewTextInput += textBox_PreviewTextInput;
            textBox.PreviewKeyDown += textBox_PreviewKeyDown;
            DataObject.AddPastingHandler(textBox, Pasting);
        }
    }

    private static void textBox_PreviewTextInput(object sender, TextCompositionEventArgs e)
    {
        var textBox = sender as TextBox;
        var maskExpression = GetMaskExpression(textBox);

        if (maskExpression == null)
        {
            return;
        }

        var proposedText = GetProposedText(textBox, e.Text);

        if (!maskExpression.IsMatch(proposedText))
        {
            e.Handled = true;
        }
    }

    private static void textBox_PreviewKeyDown(object sender, KeyEventArgs e)
    {
        var textBox = sender as TextBox;
        var maskExpression = GetMaskExpression(textBox);

        if (maskExpression == null)
        {
            return;
        }

        //pressing space doesn't raise PreviewTextInput - no idea why, but we need to handle
        //explicitly here
        if (e.Key == Key.Space)
        {
            var proposedText = GetProposedText(textBox, " ");

            if (!maskExpression.IsMatch(proposedText))
            {
                e.Handled = true;
            }
        }
    }

    private static void Pasting(object sender, DataObjectPastingEventArgs e)
    {
        var textBox = sender as TextBox;
        var maskExpression = GetMaskExpression(textBox);

        if (maskExpression == null)
        {
            return;
        }

        if (e.DataObject.GetDataPresent(typeof(string)))
        {
            var pastedText = e.DataObject.GetData(typeof(string)) as string;
            var proposedText = GetProposedText(textBox, pastedText);

            if (!maskExpression.IsMatch(proposedText))
            {
                e.CancelCommand();
            }
        }
        else
        {
            e.CancelCommand();
        }
    }

    private static string GetProposedText(TextBox textBox, string newText)
    {
        var text = textBox.Text;

        if (textBox.SelectionStart != -1)
        {
            text = text.Remove(textBox.SelectionStart, textBox.SelectionLength);
        }

        text = text.Insert(textBox.CaretIndex, newText);

        return text;
    }
}

I've improved Kent Boogaart's answer by also handling the following actions that previously could cause the pattern to be violated:

• Backspace
• Selecting and dragging text in a way that can violate the pattern
• Cut command

For example, Kent Boogaart's answer allowed the user to enter "ac" by first entering "abc" and afterwards delete the "b" with the backspace which violates the following regex

^(a|ab|abc)$

Usage (unchanged):

<TextBox b:Masking.Mask="^\p{Lu}*$"/>

Mask class:

public static class Masking
{
    private static readonly DependencyPropertyKey _maskExpressionPropertyKey = DependencyProperty.RegisterAttachedReadOnly("MaskExpression",
            typeof(Regex),
            typeof(Masking),
            new FrameworkPropertyMetadata());

    /// <summary>
    /// Identifies the <see cref="Mask"/> dependency property.
    /// </summary>
    public static readonly DependencyProperty MaskProperty = DependencyProperty.RegisterAttached("Mask",
            typeof(string),
            typeof(Masking),
            new FrameworkPropertyMetadata(OnMaskChanged));

    /// <summary>
    /// Identifies the <see cref="MaskExpression"/> dependency property.
    /// </summary>
    public static readonly DependencyProperty MaskExpressionProperty = _maskExpressionPropertyKey.DependencyProperty;

    /// <summary>
    /// Gets the mask for a given <see cref="TextBox"/>.
    /// </summary>
    /// <param name="textBox">
    /// The <see cref="TextBox"/> whose mask is to be retrieved.
    /// </param>
    /// <returns>
    /// The mask, or <see langword="null"/> if no mask has been set.
    /// </returns>
    public static string GetMask(TextBox textBox)
    {
        if (textBox == null)
        {
            throw new ArgumentNullException("textBox");
        }

        return textBox.GetValue(MaskProperty) as string;
    }

    /// <summary>
    /// Sets the mask for a given <see cref="TextBox"/>.
    /// </summary>
    /// <param name="textBox">
    /// The <see cref="TextBox"/> whose mask is to be set.
    /// </param>
    /// <param name="mask">
    /// The mask to set, or <see langword="null"/> to remove any existing mask from <paramref name="textBox"/>.
    /// </param>
    public static void SetMask(TextBox textBox, string mask)
    {
        if (textBox == null)
        {
            throw new ArgumentNullException("textBox");
        }

        textBox.SetValue(MaskProperty, mask);
    }

    /// <summary>
    /// Gets the mask expression for the <see cref="TextBox"/>.
    /// </summary>
    /// <remarks>
    /// This method can be used to retrieve the actual <see cref="Regex"/> instance created as a result of setting the mask on a <see cref="TextBox"/>.
    /// </remarks>
    /// <param name="textBox">
    /// The <see cref="TextBox"/> whose mask expression is to be retrieved.
    /// </param>
    /// <returns>
    /// The mask expression as an instance of <see cref="Regex"/>, or <see langword="null"/> if no mask has been applied to <paramref name="textBox"/>.
    /// </returns>
    public static Regex GetMaskExpression(TextBox textBox)
    {
        if (textBox == null)
        {
            throw new ArgumentNullException("textBox");
        }

        return textBox.GetValue(MaskExpressionProperty) as Regex;
    }

    private static void SetMaskExpression(TextBox textBox, Regex regex)
    {
        textBox.SetValue(_maskExpressionPropertyKey, regex);
    }

    private static void OnMaskChanged(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs e)
    {
        var textBox = dependencyObject as TextBox;
        var mask = e.NewValue as string;
        textBox.PreviewTextInput -= textBox_PreviewTextInput;
        textBox.PreviewKeyDown -= textBox_PreviewKeyDown;
        DataObject.RemovePastingHandler(textBox, Pasting);
        DataObject.RemoveCopyingHandler(textBox, NoDragCopy);
        CommandManager.RemovePreviewExecutedHandler(textBox, NoCutting);


        if (mask == null)
        {
            textBox.ClearValue(MaskProperty);
            textBox.ClearValue(MaskExpressionProperty);
        }
        else
        {
            textBox.SetValue(MaskProperty, mask);
            SetMaskExpression(textBox, new Regex(mask, RegexOptions.Compiled | RegexOptions.IgnorePatternWhitespace));
            textBox.PreviewTextInput += textBox_PreviewTextInput;
            textBox.PreviewKeyDown += textBox_PreviewKeyDown;
            DataObject.AddPastingHandler(textBox, Pasting);
            DataObject.AddCopyingHandler(textBox, NoDragCopy);
            CommandManager.AddPreviewExecutedHandler(textBox, NoCutting);
        }
    }

    private static void NoCutting(object sender, ExecutedRoutedEventArgs e)
    {
        if(e.Command == ApplicationCommands.Cut)
        {
            e.Handled = true;
        }
    }

    private static void NoDragCopy(object sender, DataObjectCopyingEventArgs e)
    {
        if (e.IsDragDrop)
        {
            e.CancelCommand();
        }
    }

    private static void textBox_PreviewTextInput(object sender, TextCompositionEventArgs e)
    {
        var textBox = sender as TextBox;
        var maskExpression = GetMaskExpression(textBox);

        if (maskExpression == null)
        {
            return;
        }

        var proposedText = GetProposedText(textBox, e.Text);

        if (!maskExpression.IsMatch(proposedText))
        {
            e.Handled = true;
        }
    }

    private static void textBox_PreviewKeyDown(object sender, KeyEventArgs e)
    {
        var textBox = sender as TextBox;
        var maskExpression = GetMaskExpression(textBox);

        if (maskExpression == null)
        {
            return;
        }

        string proposedText = null;

        //pressing space doesn't raise PreviewTextInput, reasons here http://social.msdn.microsoft.com/Forums/en-US/wpf/thread/446ec083-04c8-43f2-89dc-1e2521a31f6b?prof=required
        if (e.Key == Key.Space)
        {
            proposedText = GetProposedText(textBox, " ");
        }
        // Same story with backspace
        else if(e.Key == Key.Back)
        {
            proposedText = GetProposedTextBackspace(textBox);
        }

        if (proposedText != null && proposedText != string.Empty && !maskExpression.IsMatch(proposedText))
        {
            e.Handled = true;
        }

    }

    private static void Pasting(object sender, DataObjectPastingEventArgs e)
    {
        var textBox = sender as TextBox;
        var maskExpression = GetMaskExpression(textBox);

        if (maskExpression == null)
        {
            return;
        }

        if (e.DataObject.GetDataPresent(typeof(string)))
        {
            var pastedText = e.DataObject.GetData(typeof(string)) as string;
            var proposedText = GetProposedText(textBox, pastedText);

            if (!maskExpression.IsMatch(proposedText))
            {
                e.CancelCommand();
            }
        }
        else
        {
            e.CancelCommand();
        }
    }

    private static string GetProposedTextBackspace(TextBox textBox)
    {
        var text = GetTextWithSelectionRemoved(textBox);
        if (textBox.SelectionStart > 0 && textBox.SelectionLength == 0)
        {
            text = text.Remove(textBox.SelectionStart-1, 1);
        }

        return text;
    }


    private static string GetProposedText(TextBox textBox, string newText)
    {
        var text = GetTextWithSelectionRemoved(textBox);
        text = text.Insert(textBox.CaretIndex, newText);

        return text;
    }

    private static string GetTextWithSelectionRemoved(TextBox textBox)
    {
        var text = textBox.Text;

        if (textBox.SelectionStart != -1)
        {
            text = text.Remove(textBox.SelectionStart, textBox.SelectionLength);
        }
        return text;
    }
}