unless (($key =~ /(?:^,)|,,|[^a-zA-Z,]|(?:,$)))

##</code><code>##

 if (($key =~ /^[a-zA-Z]+(?:,[a-zA-Z]+)*$))

##</code><code>##

 my $field = qr([a-zA-Z]+);
 if (($key =~ /^$field(?:,$field)*$))