extracting a substring

[b83503104 at yahoo.com]

Hi,
I have a bunch of strings like
a53bc_531.txt
a53bc_2285.txt
...
a53bc_359.txt

and I want to extract the numbers 531, 2285, ...,359.

One thing for sure is that these numbers are the ONLY part that is
changing; all the other characters are always fixed.

I know I should use regular expressions, but I'm not familar with
python, so any quick help would help, such as which commands or idioms
to use.  Thanks a lot!