Regex for pattern that starts with ABC, then B's and/or C's, and ends with CBA

Question

Let's say I have a string that can contain only A's, B's and C's.

I have substrings of a certain pattern that I want to extract: they start with ABC, continue with a combination of B's and C's, and end with CBA.

The naive solution is to use ABC[BC]*CBA.

However, that will not cover the ABCBA string. Is there a "pythonic" way to address this, other than using | to look for two possible RE's?

Matt, you do not need to use lookarounds here, your regex is fine, the only fix to be done is to enclose the `C[BC]*` part with an optional group. — Wiktor Stribiżew, May 29 '18 at 11:32
@WiktorStribiżew Thank you very much for your solution! Are optional groups more preferable than lookaround? — MattS, May 29 '18 at 15:14
You may compare the steps: [34 (mine)](https://regex101.com/r/50I7oM/3) vs. [42 (Biffen's)](https://regex101.com/r/6UNzCc/1). — Wiktor Stribiżew, May 30 '18 at 08:09

Biffen · Answer 1 · 2018-05-29T10:40:07.617

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You can use lookarounds:

AB(?=C)[BC]*(?<=C)BA

I.e. make sure AB is followed by C and BA is preceded by C, even if they are the same C.

edited May 29 '18 at 10:40

answered May 29 '18 at 10:35

Biffen

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can be simplified with just one lookaround (the lookahead for example), because one `C` has to be consumed – Nahuel Fouilleul May 29 '18 at 10:37
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this is using extended features to RE. confirmed, does work. – Nathan McCoy May 29 '18 at 10:38
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@anubhava True. – Biffen May 29 '18 at 10:40
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Thanks, I've accepted this answer (with @anubhava 's suggestion). – MattS May 29 '18 at 10:42
@Biffen indeed i was thinking to this solution – Nahuel Fouilleul May 29 '18 at 10:44
@NahuelFouilleul There is a much simpler solution without lookarounds. See [my answer](https://stackoverflow.com/a/50582770/3832970) – Wiktor Stribiżew May 29 '18 at 11:25
@WiktorStribiżew, indeed, the advantage of yours is that it's more efficient, i didn't think of it first because it's like a `|` except one branch is empty however don't understand what op meant by "pythonic" – Nahuel Fouilleul May 29 '18 at 11:51
@NahuelFouilleul I think people (mis|over)use the word "pythonic". As far as regex is concerned, "pythonic" might mean "using the most commonly used Python `re` regex features". – Wiktor Stribiżew May 29 '18 at 11:55

Wiktor Stribiżew · Accepted Answer · 2018-05-29T12:08:51.803

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You do not need to use lookarounds, use an optional group:

ABC(?:[BC]*C)?BA

See the regex demo.

Details

ABC - an ABC substring
(?:[BC]*C)? - a non-capturing group matching 0 or more occurrences of B or C chars followed with a C letter
BA - a BA substring.

This will effectively match AB that can only be followed with C and then any number of B or C letters (but this steak of chars is optional) followed with CBA.

Note that depending on what you are doing with the pattern, a capturing group will also do, ABC([BC]*C)?BA.

edited May 29 '18 at 12:08

answered May 29 '18 at 11:03

Wiktor Stribiżew

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or also `ABC(?:[BC]*C)?BA`, maybe a bit more efficient – Nahuel Fouilleul May 29 '18 at 11:57
@NahuelFouilleul Yes, in general, the more rightmost an optional thing is, the more efficient it turns out. – Wiktor Stribiżew May 29 '18 at 12:03
because it can't backtrack before first C – Nahuel Fouilleul May 29 '18 at 12:03

Regex for pattern that starts with ABC, then B's and/or C's, and ends with CBA

2 Answers2