Check a string does not contain invalid characters

Hi Folks, I am new to php. I want to accept a character string which I will use as a unique user id. I want to check the string to ensure the user has not typed characters that I consider to be invalid as part of a user id. I was thinking of doing something along the lines of: if (strpbrk($userid, '~!@#$%^&*()_+=-{}[]\\|;\':"<>?,./`') <> null) { blah, blah, blah...... but I think that would still leave me open to control characters. I am thinking maybe I should loop through the string character by character and check it's ascii value (using ord) is within the range of a-z and A-Z ? Is this the best way of achieving this ? Is there a php command to do something similar ? I have done a few web searches and haven't come up with much. TIA, JC