regex - Match specific length x or y

Question

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)?

Answer 1

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
(?=     # 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)
(?:     # Match either
 .{8}   # 8 characters
|       # or
 .{11}  # 11 characters
)       # (End of alternation)
$       # End of string