Question

I need to build a Regex (.NET syntax) to determine if a string ends with a specific value. Specifically I need to test whether a file has a specific extension (or set of extensions).

The code I’m trying to fix was using:

.*\.(png|jpg|gif)$

which is hideously slow for failed matches in my scenario (presumably due to the backtracking.

Simply removing the .* (which is fine since the API only tests for matches and doesn’t extract anything) at the beginning makes the regex much more efficient.

It still feels like it is pretty inefficient. Am I missing something obvious here?

Unfortunately, I don’t control the API in question so I need a regex to do this even though I wouldn’t normally consider regex to be the right tool for the job.

I also did some tests using the RegexOptions.RightToLeft and found that I could squeeze a little more performance out of my test case with ^.*\.(png|jpg|gif)$, but I can’t find a way to specify the RightToLeft option within the string of the regex itself so I don’t think I can use it.

Answer 1

I don’t have access to C# so I can’t try this… but you should be able to avoid too much backtracking by forcing the engine to find the end of the string first, then matching the extensions:

$(?<=\.(gif|png|jpg))

I’m not sure of the effect the look-behind has on performance, though.