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Refer by relative way to regex group (eg. the last one)

by Anonymous Monk
on Jan 19, 2022 at 01:01 UTC ( #11140600=perlquestion: print w/replies, xml ) Need Help??

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

How to refer by relative way e.g. to get the last regex group from Perl code inserted inside regex, as
'foobar' =~ /(oo)(bar)(?{$word=${-1}})/

fails, word var. can't be bar ?

Replies are listed 'Best First'.
Re: Refer by relative way to regex group (eg. the last one)
by LanX (Sage) on Jan 19, 2022 at 01:22 UTC
      The \g{-1} syntax is interesting, I don't recall that one. When did it come about? (Of course, it's possible it's been around since Perl 4 and I just missed it :) )

      Mike

        Looks like 5.10 according to perl5100delta:

        Relative backreferences A new syntax "\g{N}" or "\gN" where "N" is a decimal integer a +llows a safer form of back-reference notation as well as allowing re +lative backreferences. This should make it easier to generate and emb +ed patterns that contain backreferences. See "Capture buffers" in perlre. (Yves Orton)

        The cake is a lie.
        The cake is a lie.
        The cake is a lie.

        Grepping the perldelta* files for \\g lists perl5100delta, so likely the feature was introduced with Perl 5.10.0.

        Syntax::Construct to the rescue :-)

        map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]

        Perl 5.9.5 if you want to be picky.

Re: Refer by relative way to regex group (eg. the last one)
by Anonymous Monk on Jan 19, 2022 at 16:10 UTC

    You do not say how your code fails, but if you add use strict; to your example you get

    $ perl -Mstrict -Mwarnings -e 'my $word; "foobar" =~ /(oo)(bar)(?{$word=${-1}})/'
    Can't use string ("-1") as a SCALAR ref while "strict refs" in use at -e line 1.
    

    Suggestions:

    $ perldoc -v '@-'
    $ perldoc -v '@+'
    

    Or, under at least Perl 5.10, you could use named captures.

      > Suggestions:

      sigh...

      Cheers Rolf
      (addicted to the Perl Programming Language :)
      Wikisyntax for the Monastery

      update

      DB<29> "ab" =~ /^((.)(.))(?{say $+;say $^N; say "$1 $2 $3\n@{^CAPTUR +E}"})/ b ab ab a b ab a b

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