A regular expression (regex) can be very useful, but can perform badly and is typically misused in situations where another solution would be better. For example, a regex is often used for email validation when there are much more reliable ways to do this.
If reusing a regex repeatedly, you may be better off compiling it for performance by specifying the RegexOptions.Compiled option in the constructor. This only helps if you're using the regex a lot and doing so involves an initial performance penalty. So, ensure that you check whether there is actually an improvement and it isn't now slower.
The RegexOptions.IgnoreCase option can also affect performance, but it may in fact slow things down, so always test for your inputs. ...
