Form_validation request: Regex matching rules

[eluser]Colin Williams[/eluser]
http://twitter.com/crynobone brought up a great idea for the Form_validation class. He proposes it support regex matching to minimize the need for callback functions. I whole-heartedly support the idea.

Two possibilities: Create a new rule "matches_regex[$exp]" or extend the existing "matches" rule to discover a regex (based on the syntax of the passed value).

Anyone up to the challenge? I'm already neck deep in developing other CI extensions but really like this idea.