XSLT: Finding last occurence in a string

Question

Given a form number like:

ABC_12345_Q-10

I want to end up with:

ABC12345

So I need to find the position o

Answer 1

Easier solution in XSLT 2.0:

codepoints-to-string(reverse(string-to-codepoints(
    substring-before(
        codepoints-to-string(reverse(string-to-codepoints($s))), '_'))))

With 'substring-before' you will get everything after the last occurrence of your delimiter (the underscore). If you use 'substring-after' instead, you will get everything before the last occurrence of your deliminator.