That seems to produce the same result, namely:

string is now Goodbye \1

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

my $string = 'Hello sailor';
my $regex = 'Hello (.*)';
my $substitution = 'Goodbye \\1';

$string =~ s/$regex/$substitution/;
print "string is now $string \n";

# I wanted:     string is now Goodbye sailor
# I got:          string is now Goodbye \1
[download]

yeah sorry I wasn't able to test from Android, the problem is that the substitution part of s/// is supposed to be a literal. Otherwise you have to apply the /e eval modifiers.

my $string = 'Hello sailor';
my $regex = 'Hello (.*)';
# my $substitution = 'Goodbye \1';

$string =~ s/$regex/Goodbye $1/;
print "string is now <$string> \n";
[download]

/usr/bin/perl -w /tmp/regex.pl 
string is now <Goodbye sailor>
[download]

Please note that \1 is somehow deprecated in the substitution part and will cause warnings.

Its only legitimate use is in the match part for backreference.

Cheers Rolf

PS: Je suis Charlie!

That's how you go if you want to hold the substitution part in a variable.

my $string = 'Hello sailor';
my $regex = 'Hello (.*)';
my $substitution = '"Goodbye $1"';

$string =~ s/$regex/$substitution/ee;
print "string is now <$string> \n";
[download]

/usr/bin/perl -w /tmp/regex.pl 
string is now <Goodbye sailor>
[download]

Cheers Rolf

PS: Je suis Charlie!

Yes, that seems to work perfectly. Thank you.