simplest way to check if a given string is a positive integer using regex

Like this (doesn't allow leading zeroes):
[1-9][0-9]*

You'll probably want to make sure you don't match on a partial string too actually, so you'll need to match the start and end of the string with ^ and $ respectively:
^[1-9][0-9]*$

Hmm, your first solution seems to work as well and doesn't appear to match on a partial string (which is as desired).   Does the need for ^ and $ depend on what language and api is being used?  I am using java and am doing

inputString.matches([1-9][0-9]*)

and it (as desired) returns false for partial matches, so for "blah45" or "45blah" it returns false, but for "45" it returns true.

Why then would we need the begin and end characters?

this is simple you don't need regular expression. convert the string to number (if necessary, if not depending on your language will do it for you automatically), then check if its greater than 0, eg  if int(string) > 0. i can't see why you need a regular expression for that simple task.

ghostdog74:  I don't want to do it via exception handling (I am using java).

You can simplify that somewhat to

boolean isPositive = s.matches("\\d+");

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CEHJ - that looks like the best solution thus far - but it doesn't exclude 0 and, as mentioned, I need 1 and up.  Any thoughts?

:-)