[rt-users] Regular expressions (Perl regex) for validation

[Gaston Huot]

I'm trying to make a validation for a custom field and obviously I'm
missing something.

In this custom field, the user has to enter:
  -either nothing
  -either a number of exactly 2 digits

I'm ok with the "exactly 2 digits": (?#2 digits. E.g.:01,11,12)^\d{2}$
but don't know how to add the possibility of entering nothing.

Someone has the answer ?

Thanks.

GH
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