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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