New on DevShed today there's the fifth part of their series looking at validation and filtering in PHP5 applications.

Among the enhancements that were introduced originally in PHP 5, there’s one powerful extension that has been all but ignored by many programmers until now, quite possibly because they weren’t aware of its existence. [...] In this fifth part of the series, I’m going to discuss the usage of the [filter] extension for checking if the value that has been assigned to a given variable follows a specified regular expression pattern.

They include examples of using the FILTER_VALIDATE_REGEX option for filter_var to check the input against a given format. You'll have to find an introduction to regular expressions someplace else, though - they just assume you know how to work with them.