in reply to Recursively executed function in regex RHS, AKA "wait, why does this work?"

Hello Cody Fendant,

choroba has explained what is happening, but recursion tends to be counter-intuitive and therefore hard to “grasp” or visualise. So I’ve rewritten your example code and added print statements in an attempt to make the logistics clearer:

#! perl
use strict;
use warnings;

my %includes =
(
    1 => join("\n", '<html>',
                    '<head>',
                    '  <include id="2">',
                    '  <title>hello world</title>',
                    '</head>',
                    '<body>',
                    '<div class="container">',
                    '  <h1>Hello world</h1>',
                    '  <include id="3">',
                    '</div>',
                    '</body>',
                    '</html>'),
    2 => '<link rel="stylesheet" href="foo.css">',
    3 => '<div id="footer">copyright <include id="4"></div>',
    4 => '2014',
);

render(1);

sub render
{
    my $id   = shift;
    my $html = $includes{$id};

    print '-' x 10, "\nrender($id) BEFORE:\n$html\n";

    $html =~ s/<include id="(\d+)">/render($1)/eg;

    print "render($id) AFTER:\n$html\n", '-' x 10, "\n";

    return $html;
}

[download]

Output:

 1:27 >perl 1066_SoPW.pl
----------
render(1) BEFORE:
<html>
<head>
  <include id="2">
  <title>hello world</title>
</head>
<body>
<div class="container">
  <h1>Hello world</h1>
  <include id="3">
</div>
</body>
</html>
----------
render(2) BEFORE:
<link rel="stylesheet" href="foo.css">
render(2) AFTER:
<link rel="stylesheet" href="foo.css">
----------
----------
render(3) BEFORE:
<div id="footer">copyright <include id="4"></div>
----------
render(4) BEFORE:
2014
render(4) AFTER:
2014
----------
render(3) AFTER:
<div id="footer">copyright 2014</div>
----------
render(1) AFTER:
<html>
<head>
  <link rel="stylesheet" href="foo.css">
  <title>hello world</title>
</head>
<body>
<div class="container">
  <h1>Hello world</h1>
  <div id="footer">copyright 2014</div>
</div>
</body>
</html>
----------

 1:27 >

[download]

If you follow this through a few times, it eventually starts to make sense (no, really!). Pay particular attention to the order in which the four calls to sub render are interleaved.

Hope that helps,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

In Section Seekers of Perl Wisdom