How to match, but not capture, part of a regex?

Question

I have a list of strings. Some of them are of the form 123-...456. The variable portion "..." may be:

the string "apple" followed by a hyphen, e.g. 123-apple-456
the string "banana" followed by a hyphen, e.g. 123-banana-456
a blank string, e.g. 123-456 (note there's only one hyphen)

Any word other than "apple" or "banana" is invalid.

For these three cases, I would like to match "apple", "banana", and "", respectively. Note that I never want capture the hyphen, but I always want to match it. If the string is not of the form 123-...456 as described above, then there is no match at all.

How do I write a regular expression to do this? Assume I have a flavor that allows lookahead, lookbehind, lookaround, and non-capturing groups.

The key observation here is that when you have either "apple" or "banana", you must also have the trailing hyphen, but you don't want to match it. And when you're matching the blank string, you must not have the trailing hyphen. A regex that encapsulates this assertion will be the right one, I think.

I have a much easier example, I have [email protected] and I want to match company in order to hand it over for further processing — Thorsten Staerk
– Thorsten Staerk, Commented Oct 29, 2021 at 11:32

001 · Accepted Answer · 2021-06-15 17:31:54Z

508

The only way not to capture something is using look-around assertions:

(?<=123-)((apple|banana)(?=-456)|(?=456))

Because even with non-capturing groups (?:…) the whole regular expression captures their matched contents. But this regular expression matches only apple or banana if it’s preceded by 123- and followed by -456, or it matches the empty string if it’s preceded by 123- and followed by 456.

Lookaround	Name	What it Does
(?=foo)	Lookahead	Asserts that what immediately FOLLOWS the current position in the string is foo
(?<=foo)	Lookbehind	Asserts that what immediately PRECEDES the current position in the string is foo
(?!foo)	Negative Lookahead	Asserts that what immediately FOLLOWS the current position in the string is NOT foo
(?<!foo)	Negative Lookbehind	Asserts that what immediately PRECEDES the current position in the string is NOT foo

edited Jun 15, 2021 at 17:31

001

13.6k5 gold badges38 silver badges72 bronze badges

answered Oct 13, 2010 at 17:58

Gumbo

657k112 gold badges792 silver badges852 bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

Ben Blank Over a year ago

+1 — In this case, you can work around that by using group 1 rather than group 0, but this is an excellent (and subtle!) distinction.

Gumbo Over a year ago

@Ben Blank: It definitely depends on how “match” and “capture” are interpreted.

Rots Over a year ago

Love look-around assertions! These work great with Ruby as well.

Amine KOUIS Over a year ago

@GiantCowFilms I checked in JS, and it's working fine, check this demo.

Eyal Over a year ago

Useful in grep . use grep -oP .

|

Georg Plaz · Accepted Answer · 2022-08-02 17:59:58Z

17

In javascript try: /123-(apple(?=-)|banana(?=-)|(?!-))-?456/

Remember that the result is in group 1

Debuggex Demo

Based on the input provided by Germán Rodríguez Herrera

edited Aug 2, 2022 at 17:59

Georg Plaz

6,0185 gold badges43 silver badges66 bronze badges

answered Apr 7, 2015 at 18:14

op1ekun

1,92819 silver badges26 bronze badges

1 Comment

Amine KOUIS Over a year ago

But the OP want to match only "apple", "banana", and "". It's not the case here. See this demo.

Thomas · Accepted Answer · 2010-10-13 18:10:52Z

10

Try:

123-(?:(apple|banana|)-|)456

That will match apple, banana, or a blank string, and following it there will be a 0 or 1 hyphens. I was wrong about not having a need for a capturing group. Silly me.

edited Oct 13, 2010 at 18:10

answered Oct 13, 2010 at 17:52

Thomas

5,0394 gold badges26 silver badges20 bronze badges

2 Comments

David Stone Over a year ago

This is not correct since it matches, for example, "123-coconut-456".

Thomas Over a year ago

Thought you wanted it more general...fixed.

posdef · Accepted Answer · 2017-04-07 09:24:59Z

6

I have modified one of the answers (by @op1ekun):

123-(apple(?=-)|banana(?=-)|(?!-))-?456

The reason is that the answer from @op1ekun also matches "123-apple456", without the hyphen after apple.

edited Apr 7, 2017 at 9:24

posdef

6,58211 gold badges50 silver badges97 bronze badges

answered Jan 5, 2017 at 10:38

Germán Rodríguez Herrera

711 silver badge5 bronze badges

Comments

slosd · Accepted Answer · 2010-10-13 17:59:34Z

4

Try this:

/\d{3}-(?:(apple|banana)-)?\d{3}/

edited Oct 13, 2010 at 17:59

answered Oct 13, 2010 at 17:50

slosd

3,5042 gold badges23 silver badges17 bronze badges

5 Comments

David Stone Over a year ago

This is not correct since it matches, for example, "123-coconut-456".

SilentGhost Over a year ago

@david: how's that different from your "banana" example?

David Stone Over a year ago

@SilentGhost: I only want to capture apple or banana or "". All other values are invalid, as I stated.

slosd Over a year ago

sry, in that case: /\d{3}-(?:(apple|banana)-)?\d{3}/

Razor Over a year ago

What this example shows is that it is possible to have a non-capturing group without using lookahead and lookbehind.

oriberu · Accepted Answer · 2020-03-24 23:20:10Z

0

A variation of the expression by @Gumbo that makes use of \K for resetting match positions to prevent the inclusion of number blocks in the match. Usable in PCRE regex flavours.

123-\K(?:(?:apple|banana)(?=-456)|456\K)

Matches:

Match 1  apple
Match 2  banana
Match 3

answered Mar 24, 2020 at 23:20

oriberu

1,21611 silver badges7 bronze badges

Comments

Rafael · Accepted Answer · 2023-10-27 09:49:36Z

0

"123-apple-456, 87568-555"

/(\d+-)(?:[a-z]*-?)*(\d+)/

\1\2

123-456, 87568-555

answered Oct 27, 2023 at 9:49

Rafael

133 bronze badges

1 Comment

Kevin Worth Over a year ago

This answer needs to be improved with any amount of explanation.

TJC Games · Accepted Answer · 2024-01-09 17:44:03Z

0

123-(?:(apple|banana)-)?456

The word in the middle is in capturing group 1 (.groups()[0]). If it doesn't exist, this returns null.

edited Jan 9, 2024 at 17:44

answered Jan 9, 2024 at 17:43

TJC Games

11 bronze badge

Comments

Utmost Creator · Accepted Answer · 2024-01-12 17:09:59Z

0

Regular expression to test:

\d{3}-(?:(apple|banana)-|)\d{3}

matches strings with three digits, followed by either "apple-", "banana-", or nothing, and ending with three digits. It captures "apple" or "banana" if present. It also captures result as empty when there is no "apple" or "banana"

Sandbox test link

Tested on the following data set:

123-apple-456
123-banana-456
123-banana456
123banana-456
123-456
123456
123-coconut-456
123-123-456
123-apple456

Found matches:

Match 1
1.  apple
Match 2
1.  banana
Match 3
1.   
Match 4
1.

edited Jan 12, 2024 at 17:09

answered Jan 12, 2024 at 17:04

Utmost Creator

1,1691 gold badge13 silver badges26 bronze badges

Comments

Maximus · Accepted Answer · 2023-05-16 22:35:04Z

-2

echo '16' | xargs | grep -oP '\d'

answered May 16, 2023 at 22:35

Maximus

11 bronze badge

1 Comment

user67275 Over a year ago

Please add some explanation for your code rather than posting code only. Additional explanation will be more helpful.

johmsp · Accepted Answer · 2016-06-24 18:43:18Z

-6

By far the simplest (works for python) is '123-(apple|banana)-?456'.

answered Jun 24, 2016 at 18:43

johmsp

2921 silver badge8 bronze badges

1 Comment

Loren Over a year ago

This would match 123-apple456 so it isn't correct.

Collectives™ on Stack Overflow

How to match, but not capture, part of a regex?

11 Answers 11

6 Comments

1 Comment

2 Comments

Comments

5 Comments

Comments

1 Comment

Comments

Comments

1 Comment

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

11 Answers 11

6 Comments

1 Comment

2 Comments

Comments

5 Comments

Comments

1 Comment

Comments

Comments

1 Comment

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related