I'd like a regex that is either X or Y characters long. For example, match a string that is either 8 or 11 characters long. I have currently implemented this like so: ^([0-9]{8}|[0-9]{11})$.
I could also implement it as: ^[0-9]{8}([0-9]{3})?$
My question is: Can I have this regex without duplicating the [0-9] part (which is more complex than this simple \d example)?
There is one way:
^(?=[0-9]*$)(?:.{8}|.{11})$
or alternatively, if you want to do the length check first,
^(?=(?:.{8}|.{11})$)[0-9]*$
That way, you have the complicated part only once and a generic . for the length check.
Explanation:
^ # Start of string
(?= # Positive lookahead, assert that the following regex can be matched here:
[0-9]* # any number of digits (and nothing but digits)
$ # until end of string
) # (End of lookahead)
(?: # Non-capturing group, match either
.{8} # 8 characters
| # or
.{11} # 11 characters
) # (End of alternation)
$ # End of string