sub is_valid_expr {
   local our $expr;
   $expr = qr!(?:\s*[a-zA-Z]+|\s*[1-9][0-9]*|\s*\((??{$expr})\s*\))(?:\s*[-+*/](?:\s*[a-zA-Z]+|\s*[1-9][0-9]*|\s*\((??{$expr})\s*\)))*!;
   return $_[0] =~ /^$expr\s*\z/;
}