testing for "EndsWith" efficiently with a Regex

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

Really, you could also just drop Regex altogether, and use String.EndsWidth, with the following :

var extensions = new String[] { ".png", ".jpg", ".gif" };
extensions.Any(ext => "something".EndsWith(ext));

I usually have the feeling that it ends up being faster to use simple string functions for cases like this rather than trying to find a clever way to use an efficient regex, in terms of runtime and/or development time, unless you are comfortable with and know what is efficient in terms of Regex.