use Perl6::Rules;

grammar Calc {
    rule number   { \d+ }
    rule op       { <[+-]> }
    rule term :w  { 
      <term> <op> <number> |
      <number> }
}

my $text = "1 + 2 - 3 + 4";

if($text =~ m/<Calc.term>/) {
    print "Well-formed!\n";
}