PHP check if string is properly formatted

Question

I have a PHP application which should require a special activation key for unlocking. So I want to create scripts which will check if $user_input is properly formatted.

An activation key will be like this:
1E63-DE12-22F3-1FEB-8E1D-D31F

Numbers and letters spacing may vary.
So activation key may be also like this or different:
1EE3-1E1C-R21D-1FD7-111D-D3Q5

1. Colons are separated with -
2. Every colon contains 4 characters which are a mix of letters and numbers

Thanks in advance for any help :)

Solution

This regex will match keys like the one you posted, and will also work if lowercase letters are included:

^([A-z0-9]){4}-([A-z0-9]){4}-([A-z0-9]){4}-([A-z0-9]){4}-([A-z0-9]){4}-([A-z0-9]){4}$

If you want only uppercase letters, change all the "z" in that regex for "Z"