regular expression doesn't match, why?

Question

I want to match all paths that:

don't start with "/foo-bar/"

or not ends with any extension (.jpg, .gif, etc)

examples:

/foo-bar/aaaa/fff will not match

/foo-bar/aaaa/fff.jpg will not match

/aaa/bbb will match

/aaaa/bbbb.jpg will not match

/bbb.a will not match

this is my regex:

^\/(?!foo-bar\/).*(?!\.).*$

but is not working, why?

thanks!

Answer 1

It is more easy to try to match what you don't want. Example with PHP:

if (!preg_match('~^/foo-bar/|\.[^/]+$~', $url))
    echo 'Valid!';

Your pattern doesn't work because of this part .*(?!\.).*$. The first .* is greedy and will take all the characters of the string until the end, after, to make the end of the pattern to succeed, the regex engine will backtrack one character (the last of the string). (?!\.).*$ will always match this last character if it is not a dot.

If you absolutely need an affirmative pattern, you can use this:

if (preg_match('~^/(?!foo-bar/)(?:[^/]*/)*+[^./]*$~', $url))
    echo 'Valid!';

Answer 2

You can try this one, which is a bit simpler and close to what you have tried:

^(?!\/foo-bar)([^\.]+)$

Live Demo