How do I create a regex to avoid a repeated number with optional hyphen?

Question

I've been stuck on this problem for days. I need a regex to validate an Id of this format:

^[0-9]{2}[-]{0,1}[0-9]{7}$

But from this pattern I have to exclude sets like:

Note that 22-2222221 is valid.

Can anyone help me? If the Id is written without the dash, I could use ^(\d)(?!\1{8})\d{8}$ but what if the hyphen is present?

Tried something like

^(\d)(?!\1{8})\d{8}$|^(\d)(?!\1{2})\d{1}-(?!\1{7})\d{7}$

but the second part does not seem to work properly.

Thanks to Wiktor for the solution. How would you adapt it also for pattern below?

^[0-9]{3}[-]{0,1}[0-9]{2}[-]{0,1}[0-9]{4}$

Probably something like this can work: ^(?!(\d)(?:-?\1)*$)\d{3}-?\d{2}-?\d{4}$

Maybe just check format with regex and then additionally check just by manually written code to exclude forbidden sets? It might not be an option, if you must do it in regex, but if otherwise, this approach might be even simpler and easier to read — Nikolay, Dec 22 '22 at 09:35

Wiktor Stribiżew · Accepted Answer · 2022-12-22T09:42:52.127

1

You can use

^(?!(\d)(?:-?\1)*$)\d{2}-?\d{7}$

Details:

^ - start of string
(?!(\d)(?:-?\1)*$) - a negative lookahead that fails the match if there is
- (\d) - a digit (captured in Group 1)
- (?:-?\1)* - zero or more sequences of an optional - and then the digit that was captured in Group 2
- $ - end of string, immediately to the right of the current location
\d{2}-?\d{7} - two digits, an optional -, and then seven digits
$ - end of string.

edited Dec 22 '22 at 09:42

answered Dec 22 '22 at 09:31

Wiktor Stribiżew

Hello, thanks for your help. This is something I tried but it doesn't validate something like 22-2222221 that is a valid input. – despaolo Dec 22 '22 at 09:35
@despaolo Fixed and the answer is updated. – Wiktor Stribiżew Dec 22 '22 at 09:44
1

Thanks a lot Wiktor, seems working but I have to test it more deeply. – despaolo Dec 22 '22 at 10:35
Can I ask how you would fit this to the same requirements? ^[0-9]{3}[-]{0,1}[0-9]{2}[-]{0,1}[0-9]{4}$ – despaolo Dec 22 '22 at 10:37
@despaolo It is the same, `^(?!(\d)(?:-?\1)*$)\d{3}-?\d{2}-?\d{4}$` – Wiktor Stribiżew Dec 22 '22 at 10:50

1 Answers1