Perl::...racters(3pm)

💅 NAME 💅

okay bestie, this page is all about Perl::Critic::Policy::RegularExpressions::ProhibitEscapedMetacharacters — a policy that says use character classes for literal meta-characters instead of escapes. It lives in the User Contributed Perl Documentation crowd, and the footer name Perl::...racters(3pm) is folded in here like a little name tag 🫶.

🫶 AFFILIATION 🫶

This Policy is part of the core Perl::Critic distribution. periodt 💖.

✨ DESCRIPTION ✨

Ever heard of leaning toothpick syndrome? That comes from writing regular expressions that match on characters that are significant in regular expressions. For example, the expression to match four forward slashes looks like:

m/\/\/\/\//;

Well, this policy doesn't solve that problem (write it as m{////} instead!) but solves a related one. As seen above, the escapes make the expression hard to parse visually. One solution is to use character classes. You see, inside of character classes, the only characters that are special are \, ], ^, and -, so you don't need to escape the others. So instead of the following loose IPv4 address matcher:

m/ \d+ \. \d+ \. \d+ \. \d+ /x;

You could write:

m/ \d+ [.] \d+ [.] \d+ [.] \d+ /x;

which is definitely more readable, if less recognizable before the publication of Perl Best Practices. (Of course, you should really use Regexp::Common::net to match IPv4 addresses!) 🦋

Specifically, this policy forbids backslashes immediately prior to the following characters:

{ } ( ) . * + ? | #

We make a special exception for $ because "/[$]/" turns into "/[5.008006/" for Perl 5.8.6. We also make an exception for ^ because it has special meaning (negation) in a character class. Finally, [ and ] are exempt, of course, because they are awkward to represent in character classes.

Note that this policy does not forbid unnecessary escaping. So go ahead and (pointlessly) escape ! characters 💀.

🎀 CONFIGURATION 🎀

This Policy is not configurable except for the standard options. no extra knobs, bestie ✨.

💀 BUGS 💀

Perl treats m/[#]/x in unexpected ways. The author thinks it's a bug in Perl itself, but isn't 100% sure that they haven't simply misunderstood...

This part makes sense:

"#f" =~ m/[#]f/x;     # match
"#f" =~ m/[#]a/x;     # no match

This doesn't:

$qr  = qr/f/;
"#f" =~ m/[#]$qr/x; # no match

Neither does this:

print qr/[#]$qr/x;  # yields '(?x-ism:[#]$qr
                                       )'

🌈 CREDITS 🌈

Initial development of this policy was supported by a grant from the Perl Foundation. iconic 💖.

👑 AUTHOR 👑

Chris Dolan <cdolan@cpan.org>

📜 COPYRIGHT 📜

Copyright (c) 2007-2023 Chris Dolan

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself. The full text of this license can be found in the LICENSE file included with this module

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okay bye bestie — this page is from perl v5.42.0, dated 2024-10-23, and filed under Perl::...racters(3pm) 💅✨