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)?

Accepted Answer

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
```

Match specific length x or y

Solution

Problem