Regex: Counted occurences in different places

Question

Is regex capable of matching patterns which might be separated by a different pattern?

E.g.: A{3}B{2} would match AAABB. How can I make it match ABABA?

As this is just an example, I am not looking for a way to match all permutations of AAABB, but to learn a generic method. :-)

You could assert 3 times an A and 2 times a B, then match `[AB]+` — The fourth bird, Aug 12 '20 at 10:56
Does this answer your question? [Regex lookahead, lookbehind and atomic groups](https://stackoverflow.com/questions/2973436/regex-lookahead-lookbehind-and-atomic-groups) — BadHorsie, Aug 12 '20 at 11:36
@BadHorsie: Great resource, thanks. I did not yet come up with a solution, but I learned something useful there. :) — Zsolt Szilagyi, Aug 12 '20 at 11:48
@BadHorsie: After accepting another answer: Yes, your link had the tools needed to solve my task. Knowing the syntax and finding a solution are different beasts though. — Zsolt Szilagyi, Aug 12 '20 at 12:08

rootkonda · Answer 1 · 2020-08-12T12:04:13.340

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(?=A{3}B{2})[AB]+

(?=A{3}B{2}) - Positive lookahead of the given string to make sure we have exactly 3 A's and 2 B's. If this results in true then it starts looking for [AB]+ . This strictly matches the AAABB.

https://regex101.com/r/6xcNb7/3

Updated as requested and this should match other permutations.

(?=(?:B*A){3}B*$)|(?=(?:A*B){2}A*$)[AB]{5}

edited Aug 12 '20 at 12:04

answered Aug 12 '20 at 11:03

rootkonda

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Thanks! Unfortunately there seems to be an issue - the Regex finds the first occurance of AAABB, and the n any subsequent As and Bs. It doesn't match ABABA or BBAAA. – Zsolt Szilagyi Aug 12 '20 at 11:47
`(?=(?:B*A){3}B*$)|(?=(?:A*B){2}A*$)[AB]{5}` - I see I came up almost same answer as Toto – rootkonda Aug 12 '20 at 12:02
@Zsolt - Updated the post. – rootkonda Aug 12 '20 at 12:06

Toto · Accepted Answer · 2020-08-12T12:12:36.157

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This does the job:

^(?=(?:B*A){3}B*$)(?=(?:A*B){2}A*$)[AB]{5}$

Demo & explanation

Explantion:

^               # beginning of string
  (?=             # positive lookahead, make sure we have after exactly 3 A:
    (?:             # non capture group
      B*              # 0 or more B
      A               # 1 A
    ){3}            # end group, must appear 3 times
    B*              # 0 or more B
    $               # end of string
  )               # end of string
  (?=(?:A*B){2}A*$)     # same explanation as above for exactly 2 B
  [AB]{5}       # we must have 5 A or B
$               # end of string

edited Aug 12 '20 at 12:12

answered Aug 12 '20 at 11:54

Toto

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Great, thanks! Maybe you could add/copy a high level explanation: As far as I understood: You first look for 3 groups of A with any(\*) Bs in between, then look for 2 groups of Bs with any(\*) As in between, and finally look for a total of 5 A-or-Bs. – Zsolt Szilagyi Aug 12 '20 at 12:06

Regex: Counted occurences in different places

2 Answers2