Using string variables containing literal escapes in a Perl substitution - regex

Question

I'm new to Perl and I found behaviour which I don't understand and can't solve.
I'm making a small find and replace program and there are some things I need to do. I have bunch of files that I need to process. Then I have a list of find / replace rules in an external text file. In replacing there I need three special things:
Replacing utf-8 characters (Czech diacritics)
Work with adding/removing lines (so working in a slurp mode)
Use a regular expressions
I want a program that works alone, so I wrote it so that it takes three arguments:
The file to work on
What to find
What to replace.
I'm sending parameters in a loop from a bash script which parse the rules list and loads other files.
My problem is when I have a "\n" string in a rules list and I send it to the Perl script. If it's in the first part of replacement (in the find section) it looks for a newline correctly, but when it's in the second part (the replace section) it just prints \n instead of a newline.
I tried hardcoding "\n" to the string right into the variable instead of passing it from the list and then it works fine.
What's the reason Perl doesn't interpret the "\n" string there, and how can I make it work?
This is my code:
list.txt - One line from the external replacement list
1\. ?\\n?NÁZEV PŘÍPRAVKU;\\n<<K1>> NÁZEV PŘÍPRAVKU;
farkapitoly.sh - The bash script for parsing list.txt and cycling through all of the files and calling the Perl script
...
FILE="/home/tmp.txt"
while read LINE
do
FIND=`echo "$LINE" | awk -F $';' 'BEGIN {OFS = FS} {print $1}'`
REPLACE=`echo "$LINE" | awk -F $';' 'BEGIN {OFS = FS} {print $2}'`
perl -CA ./pathtiny.pl "$FILE" "$FIND" "$REPLACE"
done < list.txt
...
pathtiny.pl - The Perl script for find and replace
#!/usr/bin/perl
use strict;
use warnings;
use Modern::Perl;
use utf8; # Enable typing Unicode in Perl strings
use open qw(:std :utf8); # Enable Unicode to STDIN/OUT/ERR and filehandles
use Path::Tiny;
my $file = path("$ARGV[0]");
my $searchStr = "$ARGV[1]";
my $replaceStr = "$ARGV[2]";
# $replaceStr="\n<<K1>> NÁZEV PRÍPRAVKU"; # if I hardcode it here \n is replaced right away
print("Search String:", "$searchStr", "\n");
print("Replace String:", "$replaceStr", "\n\n");
my $guts = $file->slurp_utf8;
$guts =~ s/$searchStr/$replaceStr/gi;
$file->spew_utf8($guts);
If it's important, I'm using Linux Mint 13 64-bit on VirtualBox (under Win 8.1) and I have Perl v5.14.2. Every file is UTF-8 with Linux endings.
Example files can be found on pastebin. this should end up like this.
But examples varies a lot. I need a universal solution to write down newline in a replacement string so it replaces correctly.

Answer 1

The problem is that the replacement string is read literally from the file, so if your file contains
xx\ny
then you will read exactly those six characters. Also, the replacement part of a substitution is evaluated as if it was in double quotes. So your replacement string is "$replaceStr" which interpolates the variable and goes no further, so you will again have xx\nyy in the new string. (By the way, please avoid using capital letters in local Perl identifiers as in practice they are reserved for globals such as Module::Names.)
The answer lies in using eval, or its equivalent - the /e modifier on the substitution.
If I write
my $str = '<b>';
my $r = 'xx\ny';
$str =~ s/b/$r/;
then the replacement string is interpolated to xx\ny, as you have experienced.
A single /e modifier evaluates the replacement as an expression instead of just a double-quoted string, but of course $r as an expression is xx\ny again.
What you need is a second /e modifier, which does the same evaluation as a single /e and then does an additional eval of the result on top. For this it is cleanest if you use qq{ .. } as you need two levels of quotation.
If you write
$str =~ s/b/qq{"$r"}/ee
then perl will evaluate qq{"$r"} as an expression, giving "xx\nyy", which, when evaluated again will give you the string you need - the same as the expression 'xx' . "\n" . 'yy'.
Here's a full program
use strict;
use warnings;
my $s = '<b>';
my $r = 'xx\nyy';
$s =~ s/b/qq{"$r"}/ee;
print $s;
output
<xx
yy>
But don't forget that, if your replacement string contains any double quotes, like this
my $r = 'xx\n"yy"'
then they must be escaped before putting the through the substitution as the expression itself also uses double quotes.
All of this is quite hard to grasp, so you may prefer the String::Escape module which has an unbackslash function that will change a literal \n (and any other escapes) within a string to its equivalent character "\n". It's not a core module so you probably will need to install it.
The advantage is that you no longer need a double evaluation, as the replacement string can be just unbackslash $r which give the right result if it evaluated as an expression. It also handles double quotes in $r without any problem, as the expression doesn't use double quotes itself.
The code using String::Escape goes like this
use strict;
use warnings;
use String::Escape 'unbackslash';
my $s = '<b>';
my $r = 'xx\nyy';
$s =~ s/b/unbackslash $r/e;
print $s;
and the output is identical to that of the previous code.
Update
Here is a refactoring of your original program that uses String::Escape. I have removed Path::Tiny as I believe it is best to use Perl's built-in inplace-edit extension, which is documented under the General Variables section of perlvar.
#!/usr/bin/perl
use utf8;
use strict;
use warnings;
use 5.010;
use open qw/ :std :utf8 /;
use String::Escape qw/ unbackslash /;
our #ARGV;
my ($file, $search, $replace) = #ARGV;
print "Search String: $search\n";
print "Replace String: $replace\n\n";
#ARGV = ($file);
$^I = '';
while (<>) {
s/$search/unbackslash $replace/eg;
print;
}

Answer 2

You got \n as a content of a string. (as two chacters 1: \ and second n, and not as one newline.
Perl interprets the \n as newline when it is as literal (e.g. it is in your code).
The quick-fix would be:
my $replaceStr=eval qq("$ARGV[2]"); #evaling a string causes interpreting the \n as literal
or, if you don't like eval, you can use the String-Escape cpan module. (the unbackslash function)

Answer 3

You're wanting a literal string to be treated as if it were a double quoted string. To do that you'll have to translate any backslash followed by another character.
The other experts have shown you how to do that over the entire string (which is risky since it uses eval with unvalidated data). Alternatively, you could use a module, String::Escape, which requires an install (not a high bar, but too high for some).
However, the following does a translation of the return value string itself in a safe way, and then it can be used like a normal value in your other search and replace:
use strict;
use warnings;
my $r = 'xx\nyy';
$r =~ s/(\\.)/qq{"$1"}/eeg; # Translate \. as a double quoted string would
print $r;
Outputs:
xx
yy