A standard C# System.Windows.Forms.TextBox, which changes color based on the specified Regex pattern.

RegexTextBox.cs

using System;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Text.RegularExpressions;

namespace WKCore
{
    public partial class RegexTextBox : TextBox
    {
        private string
            _RegexPattern = "";
        private Color
            _RegexColorMatch = Color.FromArgb(200, 255, 200),
            _RegexColorUnmatch = Color.FromArgb(255, 200, 200),
            _RegexColorFatal = Color.FromArgb(255, 0, 0);
        public string RegexPattern
        {
            get { return _RegexPattern; }
            set { _RegexPattern = value; Recheck(); }
        }
        public Color RegexColorMatch
        {
            get { return _RegexColorMatch; }
            set { _RegexColorMatch = value; Recheck(); }
        }
        public Color RegexColorUnmatch
        {
            get { return _RegexColorUnmatch; }
            set { _RegexColorUnmatch = value; Recheck(); }
        }
        public Color RegexColorFatal
        {
            get { return _RegexColorFatal; }
            set { _RegexColorFatal = value; Recheck(); }
        }
        public RegexTextBox()
        { this.TextChanged += new EventHandler(RegexTextBox_TextChanged); }
        void RegexTextBox_TextChanged(object sender, EventArgs e)
        { Recheck(); }
        private void Recheck()
        {
            try
            {
                this.BackColor =
                    Regex.Match(this.Text, this._RegexPattern).Success
                    ? _RegexColorMatch
                    : _RegexColorUnmatch;
            }
            catch
            { this.BackColor = _RegexColorFatal; }
        }
    }
}