question regarding form filtering

HI all, Well I have been going through various methods on filtering form data, and the one I never see is filtering form data using regular expressions, (although the html form and validition class by Manuel Lemos does seem to use them) this is the only I could find. I often see lines like (for checking a 4 character number for example): $input_value = html_entities($input_value); If (strval(intval($input_value)) && strlen($input_value) == 4) { // do something with validated data (maybe put in valid array or something) } Ok so whats wrong with good ole: If (preg_match('/^[0-9]{4}$/',trim($input_value)) { // do something with validated data (maybe put in valid array or something) } Am I going to get a performance hit if I validate all my fields with regular expressions? As I see it I am only calling one function (ok 2 with the trim()) to validate my form data. Just wondering what you all thought about these different methods, and what approach suits best a given situation.. Regards, Tim