Regex, That Matches No more Than n Occurrences

Question

I'm trying to find a Regex, that matches no more than n consecutive, say three, for example, occurrences of a character; in the case of three and character is "a":
abbaa - matches;
ammaaa - doesn't match(three consecutive a's), there is one "a", but whole expression must be discarded because three "a's"

Thank you.

Marou

What have you tried? What regex engine are you using? – Blastopore

Read some tutorials and experimenting on regex101. – Marou

For those cases where you know n is 1, a simple question mark will suffice. a? matches zero occurrences and one occurrence of the letter a. – Loupe

Answer 1

You can specify a character, or you can generalize it to all characters.
Also, not sure if you're trying to test or match the whole string, but assume
it's the whole string.

Generalized

^(?:(\w)(?!\1{2})|\W)+$

Explained

 ^                       # Beginning of string
 (?:                     # Cluster
      ( \w )                  # (1), a word character
      (?! \1{2} )             # Not 2 more of same in a sequence
   |                        # or,
      \W                      # Not a word character
 )+                      # End Cluster, 1 to many times
 $                       # End of string

You could substitute specific characters instead of \w and \W
by replacing what you want.

Example for character a it would be a and [^a].

^(?:(a)(?!\1{2})|[^a])+$

or more than one like [abc] and [^abc]

^(?:([abc])(?!\1{2})|[^abc])+$

Robinett

@TomaszMadry - Added an update note for specific character(s). – Robinett

Thanks, seems to be this:) – Marou

Answer 2

Use the {m,n} syntax (if your environment provides it, which it likely does.

{m,n} allows m to n (inclusive) occurrences of the previous character.

Examples:

/a{0,3}/ matches 0 to 3 occurrences of a.

/a{3,}/ matches 3 or more occurrences.

/a{3}/ matches exactly 3 occurrences.

In the following example I've paired the above syntax with negative lookahead and negative lookbehind.

(?<!a)a{0,3}(?!a) matches 0 to 3 occurrences of a where there is no a before or after those 0-3 occurrences.

Spam

But the first example /a{0,3}/, matches also, let's say: xxaaaad; regex101.com/r/f54igA/1 – Marou

Right, you have to be more specific with what you want to match if you want me to prescribe exact solutions. You would use the syntax I mentioned to build whatever regex you wanted. See my updated answer for another example. – Spam

Let me know if that last example is what you are looking for. I'm happy to discuss further and edit my question. Regexes are fun. – Spam

That works, cool. Interesting, that I've missed those negative looks syntax. I'm searching and reading now. Cheers. – Marou

Well, but still accepts abaaa - because of "a" in the expression; another answer seems to solve it. – Marou

It should match abaaa. It will match no more than 3 and that satisfies it. The look behind and look ahead are making sure there is not another a directly before the group of 0-3 a's. For example aaaa won't be accepted. Actually, the example in your question statement is wrong. No more than 3 consecutive means 3 in a row IS allowed. – Spam

Yes, about no more than three is a typo mistake, but problem is how to make it to not accept abaaaa - because there is more than three "a's". – Marou

Let us continue this discussion in chat. – Marou