comment on

Attempting to parse or manipulate HTML (or XML or similar) with a regular expression is nearly always a bad idea. I'm actually working at the moment and don't have time to pull out references; however, I'm sure others will do so.

From a purely academic perspective:

^ anchors to the start of $contents — that's fine.
[^<form]* is a negated, bracketed character class (zero or more times) — not what you want. See perlrecharclass for details. It would be better to use .+? (one or more of any character, non-greedily); with a /s modifier, . will also match newlines.
(?=<form) matches up to but not including "<form" — that's fine.
The /m modifier — there's nothing in your regex that makes this useful.
The /g modifier — you only want to remove content once; don't use this.
See perlre for any of the above that you haven't understood.

Here's a guess at your original content with a demonstration of your posted regex and my suggested one.

perl -E '
    my $content = q{<!DOCTYPE html>
<html>
<head>
    <title>Whatever</title>
</head>
<body>
    <h1>Heading</h1>
    <form>...</form>
</body>
</html>
};

say "Full:";
say $content;

my $contents = $content;
$contents =~ s/^[^<form]*(?=<form)//mg;
say "\nWith your s///:";
say $contents;

$content =~ s/^.+?(?=<form)//s;

say "\nShortened:";
say $content;
'
Full:
<!DOCTYPE html>
<html>
<head>
    <title>Whatever</title>
</head>
<body>
    <h1>Heading</h1>
    <form>...</form>
</body>
</html>


With your s///:
<!DOCTYPE html>
<html>
<head>
    <title>Whatever</title>
</head>
<body>
    <h1>Heading</h1>
<form>...</form>
</body>
</html>


Shortened:
<form>...</form>
</body>
</html>
[download]

For general usage when regexes aren't doing what you expected, I can highly recommend Regexp::Debugger.

— Ken

In reply to Re: Substitution remove all before by kcott
in thread Substitution remove all before by Anonymous Monk

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