You have received some good answers based on the info you have given. Unfortunately, that info has been incomplete or contradictory and has caused considerable wasted motion (and a bit of evident frustration) on the part of your fellow monks. What we'd like to know is something like this: for a typical string defined thusly
    abcd1_abc_123456.abc1a_A.201307282200-2300-12.gz
                  field A ---^^^^^^^^^^^^      ^^--- field B
             fixed width?                            variable width!
            fixed offset?                            fixed offset?
sort first (numerically? ascending?) by field X, then sort (numerically? ascending?) by field Y.

A mind is a terrible thing to waste.   Please think before you post!