In PHP how would I make a constraint on a certain field, for example a booking reference needs to start with a three letter group such as 'ZRE', 'FGH', 'TYU'. The next character must be a hyphen, the first digit after this must only be a 6 or a 9 and the rest of the 5 digits must be between 0 and 9, anything other than this format is invalid, thanks. I know how to make a constraint for letters = /^a-zA-Z]{1,30}$/ but how do i make it just certain letters like the ones above :)