Keeping it regular

Last week I posted an example of using a regular expression to control the input of numbers into a TextBox. A couple of my fellow disciples commented that they’d just use a regular expression Behavior or DP to control the input of the text. Now, there are a couple of reasons that I’d use the dedicated NumericTextBoxBehavior.

The first reason is that it’s a simple control for those that aren’t comfortable writing regular expressions. After all, why should you write a complex regular expression when I can write one for you?

The second reason is that the numeric control is internationalised from the get-go. I’ve already take care of sorting out the whole internationalised decimal place issue, so you don’t have to worry about it with your regular expression.

Saying that, the regular expression behavior is a cracking idea, and one I could kick myself for not thinking of earlier. So, in order to add regular expression functionality in your TextBox, all you need do is add the following code:

namespace Goldlight.Base.Behaviors
  using System.Linq;
  using System.Windows.Controls;
  using System.Windows.Interactivity;
  using System.Windows;
  using System.Windows.Input;
  using System.Text.RegularExpressions;

  /// <summary>
  /// Apply this behavior to a TextBox to ensure that input matches a regular
  /// expression.
  /// <para>
  /// <remarks>
  /// In the view, this behavior is attached in the following way:
  /// <code>
  /// <TextBox Text="{Binding Price}">
  ///   <i:Interaction.Behaviors>
  ///   <gl:RegularExpressionTextBoxBehavior 
  ///    Mask="^([\(]{1}[0-9]{3}[\)]{1}[ ]{1}[0-9]{3}[\-]{1}[0-9]{4})$" />
  ///   </i:Interaction.Behaviors>
  /// </TextBox>
  /// </code>
  /// <para>
  /// Add references to System.Windows.Interactivity to the view to use
  /// this behavior.
  /// </para>
  /// </remarks>
  public class RegularExpressionTextBoxBehavior : Behavior<TextBox>
    /// <summary>
    /// Gets or sets the regular expression mask.
    /// </summary>
    public string Mask { get; set; }

    #region Overrides
    protected override void OnAttached()

      AssociatedObject.PreviewTextInput += AssociatedObject_PreviewTextInput;
      DataObject.AddPastingHandler(AssociatedObject, OnClipboardPaste);

    protected override void OnDetaching()
      AssociatedObject.PreviewTextInput -= AssociatedObject_PreviewTextInput;
      DataObject.RemovePastingHandler(AssociatedObject, OnClipboardPaste);

    /// <summary>
    /// Handle paste operations into the textbox to ensure that the behavior
    /// is consistent with directly typing into the TextBox.
    /// </summary>
    /// <param name="sender">The TextBox sender.</param>
    /// <param name="dopea">Paste event arguments.</param>
    /// <remarks>This operation is only available in WPF.</remarks>
    private void OnClipboardPaste(object sender, DataObjectPastingEventArgs dopea)
      string text = dopea.SourceDataObject.GetData(dopea.FormatToApply).ToString();

      if (!string.IsNullOrWhiteSpace(text) && !Validate(text))

    /// <summary>
    /// Preview the text input.
    /// </summary>
    /// <param name="sender">The TextBox sender.</param>
    /// <param name="e">The composition event arguments.</param>
    void AssociatedObject_PreviewTextInput(object sender, TextCompositionEventArgs e)
      e.Handled = !Validate(e.Text);

    /// <summary>
    /// Validate the contents of the textbox with the new content to see if it is
    /// valid.
    /// </summary>
    /// <param name="value">The text to validate.</param>
    /// <returns>True if this is valid, false otherwise.</returns>
    protected bool Validate(string value)
      TextBox textBox = AssociatedObject;

      string pre = string.Empty;
      string post = string.Empty;

      if (!string.IsNullOrWhiteSpace(textBox.Text))
        pre = textBox.Text.Substring(0, textBox.SelectionStart);
        post = textBox.Text.Substring(textBox.SelectionStart + textBox.SelectionLength, 
          textBox.Text.Length - (textBox.SelectionStart + textBox.SelectionLength));
        pre = textBox.Text.Substring(0, textBox.CaretIndex);
        post = textBox.Text.Substring(textBox.CaretIndex, 
          textBox.Text.Length - textBox.CaretIndex);
      string test = string.Concat(pre, value, post);

      string pattern = Mask;

      if (string.IsNullOrWhiteSpace(pattern))
        return true;

      return new Regex(pattern).IsMatch(test);

As you can see, it’s similar in code to the other behaviour. The only real difference in it is that it has a Mask string which is used to add the regular expression text.

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