How to capture one of two things, then the other of two things

Question

This is a generic regular expression question, that I couldn't find an answer too.

Lets say I have a few input strings:

...A-A-B...
...B-A-A...
...A-B-A...
...B-B-A...

I have two delimiters A and B. What I want to do is find either of these, and then look for the other one. Now I know I can do (A.*B)|(B.*A) which will match what I want for all four strings.

And I can also do (A|B).*\1 which would match A-A for the first and second strings, A-B-A for the third and B-B for the last.

Can I do something alongs the lines of (A|B).*[^\1] to say, find one of two tokens then find the other token?

For context, I was interested in this because of this question, where A and B are a dot or comma for currency matching, but I figure this could be useful in lots of contexts, hence the generic question.

score 3 · Accepted Answer · answered Sep 05 '13 at 23:58

3

Negative lookahead could do it.

(A|B).*(?!\1)(A|B)

answered Sep 05 '13 at 23:58

John Kugelman

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Casimir et Hippolyte · Answer 2 · 2013-09-06T00:29:20.050

3

You can use a conditional:

(?:(A)|B).*?(?(1)B|A)

(?(1)B|A) is an IF..THEN..ELSE and means: if group 1 then B else A

Notice: this doesn't work with java, javascript and GO but works with Perl, PHP, Python, Ruby, .net

edited Sep 06 '13 at 00:29

answered Sep 06 '13 at 00:17

Casimir et Hippolyte

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Sorry, I upvoted you, but selected the other one as it works everywhere. But the conditional stuff is might cool. Thanks! – Sep 06 '13 at 02:18
@LegoStormtroopr: the only answer that works everywhere is rmmh answer. – Casimir et Hippolyte Sep 06 '13 at 02:44

How to capture one of two things, then the other of two things

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