Escaping \ in regex: Unmatched ( in regex; marked by <-- HERE
How can you check if a string contains a backslash \ or even better it starts with one?
You could use use the index function to check that, but what if this is only a part of something more complex that will require the use of regexes?
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:
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
use 5.010; use strict; use warnings; my $pat = '(\\)'; say '\\' =~ /^$pat/ ? "OK" : "NO MATCH!\n"; # Unmatched ( in regex
and suddenly we got and error:
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" on 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.
On 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.
use 5.010; use strict; use warnings; my $pat = qr'(\\)'; say '\\' =~ /^$pat/ ? "OK" : "NO MATCH!\n"; # OK