string input="828374823[938929293]";
Regex regex=newRegex(@"(.+)?\[(.+)?\]",RegexOptions.IgnoreCase);
if (regex.IsMatch(input))
{
MatchCollection matches = regex.Matches(input);
foreach(Match match in matches)
{
txtBox1.Text= match.Groups[1].Value;
txtBox2.Text=match.Groups[2].Value;
}
}