What is the regex to match a string not containing more than x consecutive characters

Question

I want to match strings that do not contain more than 3 of the same character repeated in a row. So:

• abaaaa [no match]
• abawdasd [match]
• abbbbasda

Answer 1

To match strings not containing a character repeated more than 3 times consecutively:

^((.)\2?(?!\2\2))+$

How it works:

^            Start of string
(
  (.)        Match any character (not a new line) and store it for back reference.
    \2?      Optionally match one more exact copies of that character.
    (?!      Make sure the upcoming character(s) is/are not the same character.
      \2\2   Repeat '\2' for as many times as you need
    )
)+           Do ad nauseam
$            End of string

So, the number of /2 in your whole expression will be the number of times you allow a character to be repeated consecutively, any more and you won't get a match.

E.g.

• ^((.)\2?(?!\2\2\2))+$ will match all strings that don't repeat a character more than 4 times in a row.

• ^((.)\2?(?!\2\2\2\2))+$ will match all strings that don't repeat a character more than 5 times in a row.

Please be aware this solution uses negative lookahead, but not all not all regex flavors support it.