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

User · Answer

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

User · Answer

Try this     d 3 -    apple banana -   d 3

User · Answer

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

User · Answer

Update  Thanks to Germ  n Rodr  guez Herrera   In javascript try   123- apple   -  banana   -     -  - 456   Remember that the result is in group 1  Debuggex Demo

User · Answer

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

User · Answer

The only way not to capture something is using look-around assertions      lt  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      -------------------------------------------------------------------------     lt  foo         Lookbehind   Asserts that what immediately PRECEDES the                                 current position in the string is foo      -------------------------------------------------------------------------     foo          Negative     Asserts that what immediately FOLLOWS the                    Lookahead     current position in the string is NOT foo  -------------------------------------------------------------------------     lt  foo         Negative     Asserts that what immediately PRECEDES the                   Lookbehind    current position in the string is NOT foo  -------------------------------------------------------------------------

User · Answer

By far the simplest  works for python  is  123- apple banana - 456

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

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