Regular Expressions

Optimizing Regex performance with RegexOptions.RightToLeft

Regex is fast when it is scanning text that doesn't match it's pattern at all. However, when finding text that almost matches, things can start to slow down. You really want the Regex to either match a text or discard it as soon as possible. Building up large potential matches and finding that they don't fit towards the end can end up being very costly. 

For patterns that are string literals, the Regex will run in linear time. So if you get lots of partial matches that get discarded after 20 characters and another Regex discards potential matches after 2 characters then the first will be in the order of ten times slower.