'What is the correct Raku recursive regex syntax?

What or how is Raku recursive regex syntax and all match variable in Raku as on try

'hellohelloworldworld' ~~ m{ ^(h\w+?o) (?0) world  };
say "\n=$&"

seems to not work

Please help out solving these.

raku


Solution 1:[1]

Raku has dedicated syntax for anonymous recursive regexes :<~~>.

Using this syntax, you could write the regex in your question as:

'hellohelloworldworld' ~~ m{ ^(h\w+?o) <~~>? world  };

say $/; # OUTPUT: «?hellohelloworld??
        #          0 => ?hellohello??»

Solution 2:[2]

The two answers I expected to see have already been posted, they are:

  1. " {} publication" of a match variable for use later within the same regex/matching operation (technically a backreference):
    > say $/ if 'hellohelloworldworld' ~~ m/ ^(h\w+?o) {} $0 world /;
    ?hellohelloworld?
     0 => ?hello?
    > say $/ if 'hellohelloworldworld' ~~ m/ ^(h\w+?o) world /;
    ?hellohelloworld?
     0 => ?hellohello?

and,

  1. use of Raku's dedicated " <~~> recursing-match" operator within the regex.

In true TMTOWTDI-spirit, there is however a third option, using Raku's :nd() adverb to achieve a sort of "poor-man's" recursion. Starting from the ['(' \w* ] grouping, you can successively pull out ?(bird?, ?(in?, and ?(nest? from the input string (bird(in(nest))). Or all three at once (last example):

In the Raku REPL:

> my $nested = "(bird(in(nest)))";
(bird(in(nest)))
> say $nested;
(bird(in(nest)))
> say  $nested ~~ m:1st/ ['(' \w* ] /;
?(bird?
> say  $nested ~~ m:2nd/ ['(' \w* ] /;
?(in?
> say  $nested ~~ m:3rd/ ['(' \w* ] /;
?(nest?
> say  $nested ~~ m:nd(1..3)/ ['(' \w* ] /;
(?(bird? ?(in? ?(nest?)
>

Behind the scenes this is most likely using Raku's :position adverb or :continue adverb, in conjunction with Raku's $/.to match variable:

> say  $nested ~~ m/ ['(' \w* ] /;
?(bird?
> say  $nested ~~ m:pos($/.to)/ ['(' \w* ] / given $nested ~~ m/ ['(' \w* ] /;
?(in?
> say  $nested ~~ m:pos($/.to)/ ['(' \w* ] / given $nested ~~ (m/ ['(' \w* ] / && m:pos($/.to)/ ['(' \w* ] /);
?(nest?
>

Again, Raku gives you a lot of different ways to approach the problem, which is one of the niceties of the language.

Sources

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Source: Stack Overflow

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Solution 2

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