Re: Fwd: Working with a regex using positional captures stored in avariableperl6-users@perl.org

Hi there,

On 11.03.21 17:43, William Michels wrote:
> Hi Moritz your book is mentioned below. Care to chime in? Reply to
> perl6-users <perl6-users@perl.org> .
> 
> Thx, Bill.
> W. Michels, Ph.D.
> 
> ---------- Forwarded message ---------
> From: Joseph Brenner <doomvox@gmail.com>
> Date: Thu, Mar 11, 2021 at 12:28 AM
> Subject: Working with a regex using positional captures stored in a variable
> To: perl6-users <perl6-users@perl.org>
> 
> 
> 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.

$0 is an alias for $/[0].

When the match is in a different variable, you need to access the
capture group as $match[0] instead of its alias $/[0].

Regards,
Moritz

-- 
Moritz Lenz
https://perlgeek.de/ -- https://raku.org/

• Re: Fwd: Working with a regex using positional captures stored in avariableperl6-users@perl.org by Moritz Lenz
• Re: Fwd: Working with a regex using positional captures stored in avariableperl6-users@perl.org by Joseph Brenner