<input type="text" name="phone_number" maxlength="9" eform="Phone Number::Is not a valid phone number!:#REGEX /^[0-9]{9}$"/ />
This will validate a number with exactly nine digits, no more no less, however it wouldn’t let me fill in my phone number (which has 10 digits) nor would it for a majority of other international phone numbers. This may not be of any concern to you of course
Here’s another regular expression for US style phone numbers which allows for spaces periods and ( ) brackets (and allows for 10 digit numbers).
/^\(?\d{3}\)?[-\s.]?\d{3,4}[-\s.]?\d{4}$/
