Help me with my regexp

I am trying to match filenames that look like this:

45.pdf

Or

45_2.pdf

So there is a positive integer, an optional underscore followed by another开发者_如何学编程 positive integer, a full stop and a string reperesenting an extension.

The problem is, my regexp is also matching 45_.pdf which I don't want to.

Here it is:

$aRegexp = '/[0-9]+_?[0-9]*\\.[a-z]+/';

//$aString = '55.pdf';
//$aString = '55_5.pdf';
$aString = '55_.pdf';

var_dump(preg_match($aRegexp, $aString)); // should return int(0)

Group the underscore and the second integer together, use + instead of * for the second integer to force a match, and optional-match the entire group with ?, like so:

$aRegexp = '/[0-9]+(_[0-9]+)?\\.[a-z]+/';