Because backslash \ has special meaning in strings and regexes, if we would like to tell Perl that we really mean a backs-slash, we will have to "escape" it, by using the "escape character" which happens to be back-slash itself. So we need to write two back-slashes: \\. Becasue we will also want to capture it we also put it in parentheses.

The first solution worked:

examples/regex_escaping.pl

use 5.010;
use strict;
use warnings;

say '\\' =~ /^(\\)/ ? "OK" : "NO MATCH!\n";   # OK

Escape in a variable

Then, in order to make our regex more reusable we wanted to put it in a variable

examples/regex_escaping_broken.pl

use 5.010;
use strict;
use warnings;

my $pat = '(\\)';
say '\\' =~ /^$pat/ ? "OK" : "NO MATCH!\n";   # Unmatched ( in regex

and suddenly we got and error:

examples/regex_escaping_broken.out

Unmatched ( in regex; marked by <-- HERE in m/^( <-- HERE \)/ at examples/regex_escaping_broken.pl line 6.

In order to fix this we first need to understand what happened. So we printed out the content of $pat. It has (\) in it. That helped me understand that by the time $pat arrived in the regex Perl already "ate" one of the backslashes so now the regex engine see the opening parentheses as a special character, but not the closing parenthes as it was escaped.

One solution would be to write:

my $pat = '(\\\\)';

but that can get out of hand very quickly.

Use qr to quote a regexp like string

A better way is to use qr we create $pat so Perl will already convert our expression to a compiled regex.

examples/regex_escaping_fixed.pl

use 5.010;
use strict;
use warnings;

my $pat = qr'(\\)';
say '\\' =~ /^$pat/ ? "OK" : "NO MATCH!\n";   # OK