Format number in TextBox when typing in c#

using System;
using System.Text;
using System.Text.RegularExpressions;
using System.Windows.Controls;

namespace WpfApplication1
{
    public class TextBoxCustomNumber : TextBox
    {
        private const string _GROUP_NAME_IS_NEGATIVE = "isNegative";
        private const string _GROUP_NAME_INTEGER = "integer";
        private const string _GROUP_NAME_DOT = "dot";
        private const string _GROUP_NAME_FRACTION = "fraction";
        private static readonly Regex _RegexNumber = new Regex(@"\A(?<" + _GROUP_NAME_IS_NEGATIVE + @">-)?(?<" + _GROUP_NAME_INTEGER + @">[0-9]+)*(?<" + _GROUP_NAME_DOT + @">\.)?(?<" + _GROUP_NAME_FRACTION + @">[0-9]*)\z");
        private string _oldText;

        public TextBoxCustomNumber()
        {
            TextChanged += OnTextChanged;
        }

        private void ApplyResults(string numberFormattedStr, int caretIndex)
        {
            if (!Text.Equals(numberFormattedStr))
            {
                Text = numberFormattedStr;
                CaretIndex = caretIndex;
                _oldText = numberFormattedStr;
            }
        }

        private void OnTextChanged(object sender, TextChangedEventArgs textChangedEventArgs)
        {
            var text = Text;

            // empty field
            if (string.IsNullOrEmpty(text))
            {
                ApplyResults("", 0);
                return;
            }

            // allow only numbers
            var match = _RegexNumber.Match(text.Replace(",", ""));
            if (!match.Success)
            {
                // not a number => restore old value
                ApplyResults(_oldText, CaretIndex);
                return;
            }

            // number => try to format
            var numberAndFormattedStr = ConstructAndFormat(
                    match.Groups[_GROUP_NAME_IS_NEGATIVE].Success,
                    match.Groups[_GROUP_NAME_INTEGER].Success ? match.Groups[_GROUP_NAME_INTEGER].Value : "",
                    match.Groups[_GROUP_NAME_DOT].Success,
                    match.Groups[_GROUP_NAME_FRACTION].Success ? match.Groups[_GROUP_NAME_FRACTION].Value : ""
                    );
            var number = numberAndFormattedStr.Item1;
            var numberFormattedStr = numberAndFormattedStr.Item2;

            // calculate caret index
            // ...

            ApplyResults(numberFormattedStr, CaretIndex);
        }

        private static Tuple<decimal?, string> ConstructAndFormat(bool isNegative, string integerStr, bool dot, string fractionStr)
        {
            var numberStr = (isNegative ? "-" : "") + integerStr + (dot ? "." : "") + fractionStr;

            decimal numberAbs;
            if (!decimal.TryParse(numberStr, out numberAbs))
            {
                return new Tuple<decimal?, string>(null, numberStr);
            }
            numberAbs = Math.Abs(numberAbs);

            var sbFractionFormatter = new StringBuilder();
            for (var i = 0; i < fractionStr.Length; i++)
            {
                sbFractionFormatter.Append('0');
            }

            var numberFormattedStr = string.Format("{0}{1:#,##0." + sbFractionFormatter.ToString() + "}{2}",
                isNegative ? "-" : "",
                numberAbs,
                string.IsNullOrEmpty(fractionStr) && dot ? "." : ""
                );

            return new Tuple<decimal?, string>(numberAbs, numberFormattedStr);
        }
    }
}

TextChanged += (sender, args) =>
{
    try
    {
        int iKeep = SelectionStart - 1;
        for (int i = iKeep; i > 0; i--)
            if (Text[i] == ',')
                iKeep -= 1;

        Text = String.Format("{0:N0}", Convert.ToInt32(Text.Replace(",", "")));
        for (int i = 0; i < iKeep; i++)
            if (Text[i] == ',')
                iKeep += 1;

        SelectionStart = iKeep + 1;
    }
    catch
    {
        //errorhandling
    }
};