I was playing around with some code to render a kind of templated HTML, and I came up with the code below.

The HTML contains special markup, <include id="n"> where n is an integer. These are looked up in a db but here they're just returned by a sub to make this question simple and self-contained. In the example below, there are nested includes where include 3 contains a reference to include 4 and 4 contains a reference to 5.

I did the rendering by re-calling the rendering function, inside the rendering function as the right-hand side of a regular expression.

My question is—why does this work? How come it renders all the way down instead of stopping after the first level? I thought it would need some kind of while loop to keep it rendering until it had processed all possible nested includes.

#!/usr/local/bin/perl
print render(1);

sub render {
    my $id = shift;
    $HTML = return_html($id);
    $HTML =~ s/<include id="(\d+)">/render($1)/eg;
    return $HTML;
}

### this sub is only here to mimic database lookups

sub return_html {
    my $id = shift;
    if ( $id == 1 ) {
        return '<html>
<head>
  <include id="2">
  <title>hello world</title>
</head>
<body>
<div class="container">
  <h1>Hello world</h1>
<include id="3">
</div>
</body>
</html>';
    }
    if ( $id == 2 ) {
        return '<link rel="stylesheet" href="foo.css">';
    }
    if ( $id == 3 ) {
        return '<div id="footer">copyright <include id="4"></div>';
    }
    if ( $id == 4 ) {
        return '2014 <include id="5">';
    }
    if ( $id == 5 ) {
        return 'and 2015';
    }

}

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

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