sub val_alphanum {
   my $self = shift;
   my ($mand, $len, $value) = @_;
   if (!$value && $mand) {
      return (undef, { msg => 'cannot be blank' });
   } elsif ($len && (length($value) > $len) ) {
    return (undef, { msg => 'is limited to '.$len.' characters.' });  
   } elsif ($value && $value !~ /^(\w*)$/) {
      return (undef, { msg => 'can only use letters, numbers and _'  
   } else {
      my $tf = new HTML::TagFilter;
      return ($tf->filter($1));
   }
}