How to escape plain text looking like variable interpolation in Perl regular expressions?

Question

I have the following lines of text in some configuration file, need to parse that file using Perl, find the File: line and replace its content using some regular expression to add some text etc.

File: logs/${byYearMonth}.log

The problem I have is with the regular expression, because I would like to use ${byYearMonth}.log in it, as it's easy to read, can easily be quoted etc. But this looks like variable interpolation to Perl and I get the following compilation error for the following simplified reg exp:

... =~ s/...\Q${byYearMonth}.log\E.../.../m;

Global symbol "$byYearMonth" requires explicit package name (did you forget to declare "my $byYearMonth"?)

Of course I can rework the reg exp to not let Perl think it's a variable name, but the provided version above is the most easiest to read and search for in my opinion. Thinking and researching about the problem, I didn't find any solution which would allow me to keep the reg exp as is and prevent the Perl compilation error by only adding some flag or whatever.

So, is there any (easy) way to tell Perl that some plain and already quoted text is not to be interpolated to get rid of the compilation error?

I have the feeling I'm missing something really easy, so thanks for your hints! :-)

Answer 1

The \Q and \E are for quoting regex meta characters. They have nothing to do with variable interpolation.

If you construct your pattern with qr'' with single quotes '', Perl will not interpolate variables.

use feature 'say';

my $foo = 1;
my $bar = qr/$foo/;
say $bar;

Output:

(?^:1)

But with qr'':

my $foo = 1;
my $bar = qr'$foo';
say $bar;

Output:

(?^:$foo)

Answer 2

I am going to call the other answers wrong, on the basis that manipulating what looks a lot like XML via regex is a bad idea.

And if it's actually not XML, then using a data format that looks like XML but isn't is an even worse idea.

So the answer is 'use a parser' (or alternatively, hit whoever generated the file with a rolled up copy of the XML spec).

Something like this will change the contents of the <File> element:

#!/usr/bin/perl
use strict;
use warnings;

use XML::Twig;

my $xml = XML::Twig -> new -> parsefile ( 'your_file.xml'); 

my $file_elt = $xml -> get_xpath ('//File',0);

print "Original value:", $file_elt -> text,"\n";
$file_elt -> set_text('some/other/path/${byWeek}.log');

$xml -> set_pretty_print ( 'indented' ); 
$xml -> print;

Note - get_xpath only finds the first instance of <File> anywhere in the tree. If you need to be more specific, you can either iterate or add additional qualifiers to the xpath.

So for example:

my $target_text = quotemeta '${byYearMonth}'; 
my $search_regex = qr/$target_text/; 

foreach my $file_elt ( $xml -> get_xpath('//File') ) { 
  if ( $file_elt -> text =~ /$search_regex/ ) { 
      ## set it to something else. 
  }
}

Answer 3

Escape the pattern with quotemeta and then compile the regex. Something like this, it works...

my $s = q[<timestamp key="byYearMonth" datePattern="yyyy-MM" />
<appender name="FILE" class="ch.qos.logback.core.FileAppender">
    <File>logs/${byYearMonth}.log</File>
    <Append>true</Append>

    <encoder class="ch.qos.logback.classic.encoder.PatternLayoutEncoder">
        <Pattern>%date %-5level %logger.%M: %msg%n</Pattern>
    </encoder>
</appender>];

my $pat = quotemeta('${byYearMonth}.log');
my $re = qr[$pat];

$s =~ s/$re/pop/g;

print $s;