Accessing captures from another capture fails a match

[p6rt]

Migrated from rt.perl.org#132120 (status was 'rejected')

Searchable as RT132120$

[p6rt]

From @zoffixznet

IRC​: https://irclog.perlgeek.de/perl6/2017-09-18#i_15181575

We can use $0 here to refer to previous capture and regex matches​:
  m​: say "11" ~~ /(\d)[$0]/
  rakudo-moar 48a84d​: OUTPUT​: «｢11｣␤ 0 => ｢1｣␤»

But if we capture that match, the match fails​:
  m​: say "11" ~~ /(\d)([$0])/
  rakudo-moar 48a84d​: OUTPUT​: «Nil␤»

[p6rt]

From @smls

On Mon, 18 Sep 2017 09​:00​:54 -0700, cpan@​zoffix.com wrote​:

IRC​: https://irclog.perlgeek.de/perl6/2017-09-18#i_15181575

We can use $0 here to refer to previous capture and regex matches​:
m​: say "11" ~~ /(\d)[$0]/
rakudo-moar 48a84d​: OUTPUT​: «｢11｣␤ 0 => ｢1｣␤»

But if we capture that match, the match fails​:
m​: say "11" ~~ /(\d)([$0])/
rakudo-moar 48a84d​: OUTPUT​: «Nil␤»

Not a bug.

Each `( )` capture represents a subpattern¹ that builds its own Match object, and inside the subpattern, `$/` refers to this subpattern's Match (which in your case doesn't have any captures) and not to the parent pattern's Match (which has the capture you meant to access).
(PS​: Remember that `$0` is just an alias for `$/[0]`.)

I don't think there's a built-in way to access a parent pattern's Match object - you'll have to manually pass on the object in a variable or similar, like you already discovered in the linked IRC log.

This ticket can IMO be closed as 'rejected', but I'll leave it open for now in case I missed something obvious or in case you want to turn it into an RFC or similar.

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[1] http://design.perl6.org/S05.html#Subpattern_captures

[p6rt]

The RT System itself - Status changed from 'new' to 'open'

[p6rt]

@zoffixznet - Status changed from 'open' to 'rejected'

[p6rt]

From @zoffixznet

On Mon, 18 Sep 2017 09​:46​:50 -0700, smls75@​gmail.com wrote​:

This ticket can IMO be closed as 'rejected', but I'll leave it open
for now in case I missed something obvious or in case you want to turn
it into an RFC or similar.

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[1] http://design.perl6.org/S05.html#Subpattern_captures

I just didn't know of this feature. Thanks for clarifying.

Rejecting