use strict;
my $str = 'https://boards.4chan.org/mlp/thread/5
http://boards.4chan.org/b/thread/570277846
https://boards.4chan.org/mlp/thread/19865655#q19865655
https://boards.4chan.org/mlp/thread/19775771
https://boards.4chan.org/co/thread/65737750#q65737750';
my $regex = qr/http(?:s)?\://boards\.4chan\.org/([a-z]*)/thread/([0-9]*)(?:\#[0-9a-z]*)?/p;
my $subst = 'Thread $2 on mlp';
my $result = $str =~ s/$regex/$subst/rg;
print "The result of the substitution is' $result\n";
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Perl, please visit: http://perldoc.perl.org/perlre.html