var s = "long string.....24X10 .....1X3";
var match = Regex.Match(s, @"\d+X\d+");
if (match.Success) {
Console.WriteLine(match.Index); // 16
Console.WriteLine(match.Value); // 24X10;
}
Also take a look at NextMatch
which is a handy function
match = match.NextMatch();
match.Value; // 1X3;