Password Strength Check

I want to require that a user choose a password that contains at least 6 characters, at least 2 of which must be letters (upper of lower case) and 2 must be non-characters (numbers or special characters). I'm having a tough time figuring out how to do this. I can use substr_count to check for the number of occurences of a character, but I don't want to check for each letter, each number and each special character for this check. Is there anything that can be used like the ereg option that use [0-9] and [a-zA-Z]? I originally was using ereg, but couldn't figure out how to make that do what I wanted as I don't care what order the letter, number and special characters are in within the password. Bottom line- if someone types in what they want to change their password to, how can I check it to ensure that it meets the requirements above?