.+(?<=\])(.+?)(?=\[\/).+

##</code><code>##

#!/usr/bin/perl -w
use strict;

my $bold = "[b]bold text[/b]";
my $red = "[color=Red]Red text text[/color]";
my $red_bold = "[color=Red][b]Red bold text[/b][/color]";

my $regex = qr/.+(?<=\])(.+?)(?=\[\/).+/;

$bold =~ s/$regex/$1/;
$red =~ s/$regex/$1/;
$red_bold =~ s/$regex/$1/;

print "\$bold: $bold\n";
print "\$red: $red\n";
print "\$red_bold: $red_bold\n";

__END__
__OUTPUT__
$bold: bold text
$red: Red text text
$red_bold: Red bold text