my site has been getting hundreds of spam messages through its contact form. These are the typical spam sent by bots, like "cheap oakley glasses buy cheap glasses online. have you ever wondered where to buy cheap oakley glasses online?" my contact form has a phone number field, and upon looking at hundreds of these messages, I noticed they all have something in common: the phone number field is filled out with a url. so using php regex, I banned letters in the phone number field. only numbers and dashes are allowed, no letters. if someone enters anything that doesn't look like a phone number and presses submit, the form will give an error, saying "your phone number does not seem to be valid. only numbers and dashes are allowed" Now, here comes the question. is this a permanent solution? or will the bots somehow "learn" what is allowed in that field, and start filling it out with numbers?