Validating regular expression without catching exception?

Question

In a .NET application (written in C#) I have a UI with a list and a text box used to enter a filter string to filter what is displayed in the list. I refresh the filter as the user types.

I now want to allow use of regular expressions in the filter string, which poses a problem. As the user types the expression it may be invalid, for instance between the time that an opening bracket is typed and the time that the closing bracket is typed. Newing up a Regex with an invalid expression throws an exception and I want to prevent this. One way is to catch the exception but I was wondering if there is some way to check that the expression is a valid regular expression without actually trying to new up a Regex with it.

Answer 1

I would have a special start character that the user types to indicate they are starting a regex, at that point you can stop the live processing of the typed text until a special ending character is typed, at which point you new up and apply the regex.

Answer 2

Just catch the exception. Given that this will be happening in response to user input there will be absolutely no significant performance penalty.