XF 1.1 Need code format to accept PayPal order numbers but no other

[HydraulicJack]

I need the regular expression that will REQUIRE the registrant to provide a string of alphanumeric characters that is exactly 17 characters in length with no spaces of special characters.

I'm about to upgrade to 1.4 when it releases, but meanwhile would like to implement this required field in my current installation to avoid having to manually delete a lot of spammers trying to register that is requiring me to manually delete them, rather than software automatically rejecting them.

I've already got the field created, but need the regex described above.

Thanks!

 

[Jake Bunce]

^[a-zA-Z0-9]{17}$

17 alphanumeric characters.

 

[HydraulicJack]

Thanks VERY much Jake!

Hopefully this won't affect people who've already registered and are already a Registered user.
If there's anything else I need to do to grandfather the previously enrolled users so this new filter doesn't exclude them, please let me know.
All the best,
Jack