Question

I’ve seen other posts on SO for RegEx matches excluding strings (using negative look ahead / look behind) but i’m still having a hard time getting it to work. Was hoping someone could help.

I have a RegEx

\.(gif|jpg|png)$

Which i use to match any filenames ending in (gif/jpg/png)

However i want to use an exclusion list so that certain file names will not be matched.

eg

• match any http://example.com/images/filename.jpg / gif / png
• except (http://example.com/images/foo.jpg, http://example.com/images/foo.gif, http://example.com/images/foo.png) (http://example.com/images/bar.jpg, http://example.com/images/bar.gif, http://example.com/images/bar.png)

Thanks very much!

Answer 1

If the number of exclusions is low, you can use multiple negative look-behinds like this:

(?<!\/foo)(?<!\/bar)(?<!www\.site\.com\/foobar)\.(gif|jpg|png)$

The ‘/’ before foo and bar makes sure the full name matches foo or bar, it doesn’t only end in it. If it is possible that there is no ‘/’ before the filename, you need to adjust this part.

One drawback of look-behinds is that you can only use a defined length, so no *, + or {1,5}is allowed. You have to specify every exception by itself.

Note that look-arounds don’t change the ‘position’ in the string from which it is looked at, which is why you can concatenate them like that.