my $AUBBC_TAGS = join '|', @AUBBC_TAGS;

my $message = '[b] stuff [i] stuff [/i] [/b]';

$message =~ s{\[(.?)?($AUBBC_TAGS)\]}{<$1$2>}xgo;
#        or s{\[(/?(?:$AUBBC_TAGS))\]}{<$1>}xgo;

print $message;