How can I prevent Perl from interpreting double-backslash as single-backslash character?

Question

How can I print a string (single-quoted) containing double-backslash \\\\ characters as is without making Perl somehow interpolating it to single-slash \\

Answer 1

Since the backslash interpolation happens in string literals, perhaps you could declare your literals using some other arbitrary symbol, then substitute them for something else later.

my $string = 'a!!!b';
$string =~ s{!}{\\}g;
print $string; #prints 'a\\\b'

Of course it doesn't have to be !, any symbol that does not conflict with a normal character in the string will do. You said you need to make a number of strings, so you could put the substitution in a function

sub bs {
    $_[0] =~ s{!}{\\}gr
}

my $string = 'a!!!b';
print bs($string); #prints 'a\\\b'

P.S. That function uses the non-destructive substitution modifier /r introduced in v5.14. If you are using an older version, then the function would need to be written like this

sub bs {
    $_[0] =~ s{!}{\\}g;
    return $_[0];
}

Or if you like something more readable

sub bs {
    my $str = shift;
    $str =~ s{!}{\\}g;
    return $str;
}

Answer 2

If you are mildly crazy, and like the idea of using experimental software that mucks about with perl's internals to improve the aesthetics of your code, you can use the Syntax::Keyword::RawQuote module, on CPAN since this morning.

use syntax 'raw_quote';
my $string1 = r'a\\\b';
print $string1; # prints 'a\\\b'

Thanks to @melpomene for the inspiration.

Answer 3

The only quoting construct in Perl that doesn't interpret backslashes at all is the single-quoted here document:

my $string1 = <<'EOF';
a\\\b
EOF
print $string1; # Prints a\\\b, with newline

Because here-docs are line-based, it's unavoidable that you will get a newline at the end of your string, but you can remove it with chomp.

Other techniques are simply to live with it and backslash your strings correctly (for small amounts of data), or to put them in a __DATA__ section or an external file (for large amounts of data).