Form validation with native PHP function

[eluser]danmontgomery[/eluser]
http://ellislab.com/codeigniter/user-gui...eppingdata

Any native PHP function that accepts one parameter can be used as a rule, like htmlspecialchars, trim, MD5, etc.

If you need to validate with these rules, like Boris Strahija said, you can create a callback to do so.