If you want to group some items in a regular expression, but don't want to have save it for later use, put a ?: after the (

eg:
source text: test1.gif test2.png test3.jpg

To match a filename with only gif and png extensions:

/(.*?\.(?:gif|png))/ 

this wil match : test1.gif, test2.png

without the ?: this would match: test1.gif, gif, test2.png, png