Working with a regex using positional captures stored in a variable

[Joseph Brenner]

Does this behavior make sense to anyone?  When you've got a regex
with captures in it, the captures don't work if the regex is
stashed in a variable and then interpolated into a regex.

Do capture groups need to be defined at the top level where the
regex is used?

{ #  From a code example in the "Parsing" book by Moritz Lenz, p. 48,
section 5.2
   my $input = 'There are 9 million bicycles in beijing.';
   if $input ~~ / (\d+) \s+ (\w+) / {
       say $0.^name;  # Match
       say $0;        # ｢9｣
       say $1.^name;  # Match
       say $1;        # ｢million｣
       say $/;
        # ｢9 million｣
        #  0 => ｢9｣
        #  1 => ｢million｣
   }
}

say '---';

{ # Moving the pattern to var which we interpolate into match
   my $input = 'There are 9 million bicycles in beijing.';
   my $pattern = rx{ (\d+) \s+ (\w+) };
   if $input ~~ / <$pattern> / {
       say $0.^name;  # Nil
       say $0;        # Nil
       say $1.^name;  # Nil
       say $1;        # Nil
       say $/;        # ｢9 million｣
   }
}

In the second case, the match clearly works, but it behaves as
though the capture groups aren't there.


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