Medhi is right -
\d*
will work - however, I would suggest
\d+
instead as it requires a minimum of one digit in order to match (\d* will match "A12" as three matches: NULL, "12", NULL - \d+ will only find the "12").
If you are going to use Regexes, then get a copy of
Expresso [
^] - it's free, and it examines and generates Regular expressions.