CODEWITHSUNDEEP
regex
David Stone
David Stone

Reputation: 3341

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

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

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.

Upvotes: 334

Views: 310696

Answers (11)

Utmost Creator
Utmost Creator

Reputation: 1121

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.

Upvotes: 0

TJC Games
TJC Games

Reputation: 1

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

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

Upvotes: 0

Rafael
Rafael

Reputation: 5

"123-apple-456, 87568-555"

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

\1\2

123-456, 87568-555

Upvotes: 0

Maximus
Maximus

Reputation: 1

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

Upvotes: 0

op1ekun
op1ekun

Reputation: 1928

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

Upvotes: 17

Gumbo
Gumbo

Reputation: 655697

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

Upvotes: 502

oriberu
oriberu

Reputation: 1216

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

Upvotes: 0

Germ&#225;n Rodr&#237;guez Herrera
Germ&#225;n Rodr&#237;guez Herrera

Reputation: 71

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.

Upvotes: 6

johmsp
johmsp

Reputation: 294

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

Upvotes: -5

Thomas
Thomas

Reputation: 4999

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.

Upvotes: 10

slosd
slosd

Reputation: 3494

Try this:

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

Upvotes: 4

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