use strict;
my $str = 'This [is a](https://google.com/) text.
This [is a](https://www.google.com/) text.
This [is a](http://google.com/) text.
This [is a](https:\\\\google.com\\) text.
This [is a](//google.com/) text.
This [is a](https://google./) text.
This [is a](https://.com/) text.
This [is a](https://googlecom/) text.
This [is a](https://google.com) text.';
my $regex = qr/\[(.+?)\]\(((?:https?:)?(\/|\\)\3(?:www\.)?.+?\.[a-z]+\3)\)/p;
my $subst = '<a href="$2">$1</a>';
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