ext.cs

public static class MyExtensions {
    public static MatchCollection Find(
    this TXTextControl.TextControl tx,
    string pattern,
    RegexOptions regexOptions) {

      // replace the CRLFs with LFs to be compatible with
      // TextControl's index
      var sText = tx.Text.Replace("\r\n", "\n");

      // new RegEx and return the matches
      Regex rg = new Regex(pattern, regexOptions);
      return rg.Matches(sText);
    }
}