Regex that gets the last set of numbers from a string

Question

"abc_d1.txt" should get 0
"abc_d1_2.txt" should get 2
"abc_d1_14.txt" should get 14
"abc_d12_x2_156.txt" should get 156

This is what I've done so far, but I am not getting the right result.

  string y = "tester_yg1.txt";
  string pattern = @"(\d+)(?!.*\d)";
  Regex rg = new Regex(pattern);
  var z = rg.Match(fullFileName).Value;
  Console.WriteLine($"z is {z}");

Answer 1

Another option is to use a single set of positive lookarounds

(?<=_[^\W\d_]\d+_?)\d*(?=\.\w+$)

Explanation

• (?<= Positive lookbehind, assert what is directly to the left is
  • _[^\W\d_]\d+_? Match _, a word char except digits or _ and 1+ digits followed by an optional _
• ) Close lookbehind
• \d* Match 0+ digits (to also get the position when there is no digit)
• (?= Positive lookahead, assert what is directly to the right is
  • \.\w+$ Match a . and 1+ word characters till the end of string
• ) Close lookahead

.NET regex demo