Preserving backslashes in Perl strings - perl

Question

Is there a way in Perl to preserve and print all backslashes in a string variable?
For example:
$str = 'a\\b';
The output is
a\b
but I need
a\\b
The problem is can't process the string in any way to escape the backslashes because
I have to read complex regular expressions from a database and don't know in which combination and number they appear and have to print them exactly as they are on a web page.
I tried with template toolkit and html and html_entity filters. The only way it works so far is to use a single quoted here document:
print <<'XYZ';
a\\b
XYZ
But then I can't interpolate variables which makes this solution useless.
I tried to write a string to a web page, into file and on the shell, but no luck, always one backslash disappears. Maybe I am totally on the wrong track, but what is the correct way to print complex regular expressions including backslashes in all combinations and numbers without any changes?
In other words:
I have a database containing hundreds of regular expressions as string data. I want to read them with perl and print them on a web page exatly as they are in the database.
There are all the time changes to these regular expressions by many administrators so I don't know in advance how and what to escape.
A typical example would look like this:
'C:\\test\\file \S+'
but it could change the next day to
'\S+ C:\\test\\file'
Maybe a correct conclusion would be to escape every backslash exactly one time no matter in which combination and in which number it appears? This would mean it works to double them up. Then the problem isn't as big as I feared. I tested it on the bash and it works with two and even three backslashes in a row (4 backslaches print 2 ones and 6 backslashes print 3 ones).

Answer 1

The backslash only has significance to Perl when it occurs in Perl source code, e.g.: your assignment of a literal string to a variable:
my $str = 'a\\b';
However, if you read data from a file (or a database or socket etc) any backslashes in the data you read will be preserved without you needing to take any special steps.

Answer 2

my $str = 'a\\b';
print $str;
This prints a\\b.
Use
my $str = 'a\\\\b';
instead

Answer 3

It's a PITA, but you will just have to double up the backslashes, e.g.
a\\\\b
Otherwise, you could store the backslash in another variable, and interpolate that.

Answer 4

The minimum to get two slashes is (unfortunately) three slashes:
use 5.016;
my $a = 'a\\\b';
say $a;

Answer 5

The problem I tried to solve does not exist. I confused initializing a string directly in the code with using the html forms. Using a string inside the code preserving all backslashes is only possible either with a here document or by reading a textfile containing the string. But if I just use the html form on a web page to insert a string and use escapeHTML() from the CGI module it takes care of all and you can insert the most wired combinations of special characters. They all get displayed and preserved exactly as inserted. So I should have started directly with html and database operations instead of trying to examine things first
by using strings directly in the code. Anyway, thanks for your help.

Answer 6

You can use the following regular expression to form your string correctly:
my $str = 'a\\b';
$str =~ s/\\/\\\\/g;
print "$str\n";
This prints a\\b.
EDIT:
You can use non-interpolating here-document instead:
my $str = <<'EOF';
a\\b
EOF
print "$str\n";
This still prints a\\b.

Answer 7

Grant's answer provided the hint I needed. Some of the other answers did not match Perl's operation on my system so ...
#!/usr/bin/perl
use warnings;
use strict;
my $var = 'content';
print "\'\"\N{U+0050}\\\\\\$var\n";
print <<END;
\'\"\N{U+0050}\\\\\\$var\n
END
print '\'\"\N{U+0050}\\\\\\$var\n'.$/;
my $str = '\'\"\N{U+0050}\\\\\\$var\n';
print $str.$/;
print #ARGV;
print $/;
Called from bash ... using the bash means of escaping in quotes which changes \' to '\''.
jamie#debian:~$ ./ft.pl '\'\''\"\N{U+0050}\\\\\\$var\n'
'"P\\\content
'"P\\\content
'\"\N{U+0050}\\\$var\n
'\"\N{U+0050}\\\$var\n
\'\"\N{U+0050}\\\\\\$var\n
The final line, with six backslashes in the middle, was what I had expected. Reality differed.
So:
"in here \" is interpolated
in HEREDOC \ is interpolated
'in single quotes only \' is interpolated and only for \ and ' (are there more?)
my $str = 'same limited \ interpolation';
perl.pl 'escape using bash rules' with #ARGV is not interpolated