Regular expressions are a powerful tool for validating inputs, but what if your input is itself a regular expression? Is there a regular expression that can validate regular expressions?

Well, yes, if your regular expression engine supports recursion: /^((?:(?:[^?+*{}()[\]\\|]+|\\.|\[(?:\^?\\.|\^[^\\]|[^\\^])(?:[^\]\\]+|\\.)*\]|\((?:\?[:=!]|\?<[=!]|\?>)?(?1)??\)|\(\?(?:R|[+-]?\d+)\))(?:(?:[?+*]|\{\d+(?:,\d*)?\})[?+]?)?|\|)*)$/.

Todays Representative Line (which is more than a single line) comes from Ryan S, who found an implementation of isValidRegex which is perhaps a bit more elegant:

	public static bool isValidRegEx(string value)
        {
            // intent is to block empty strings from being accepted
            return !string.IsNullOrEmpty(value);
        }

You might be thinking, That doesnt validate anything at all!, but at least it doesnt summon dread Cthulhu from Rgexyleh. I count that as a win.

http://thedailywtf.com/articles/the-validation-regex