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

concat(
    substring-before($s, '_'),
    substring-before(substring-after($s, '_'), '_')
)

Alternatively:

string-join(tokenize($s, '_')[position() <= 2], '')