Question

I’m trying to make a single pattern that will validate an input string. The validation rule does not allow any character to be repeated more that 3 times in a row.

For example:

Aabcddee – is valid.

Aabcddde – is not valid, because of 3 d chracters.

The goal is to provide a RegExp pattern that could match one of above examples, but not both. I know I could use back-references such as ([a-z])\1{1,2} but this matches only sequential characters. My problem is that I cannot figure out how to make a single pattern for that. I tried this, but I don’t quite get why it isn’t working:

^(([a-z])\1{1,2})+$

Here I try to match any character that is repeated 1 or 2 times in the internal group, then I match that internal group if it’s repeated multiple times. But it’s not working that way.

Thanks.

Answer 1

From your question I get that you want to match

• only strings consisting of chars from [A-Za-z] AND
• only strings which have no sequence of the same character with a length of 3 or more

Then this regexp should work:

^(?:([A-Za-z])(?:(?!\1)|\1(?!\1)))+$

(Example in perl)