grammar assertions

assertion-example.txt

# code fragment in rakudo's src/Perl6/Grammar.nqp

token numbered-alias { '#' }

my $*USE-HASH-MARK := 1; # default, means we're free to extract the first '#' in a pod block's data

# inside a pod block, e.g., abbreviated, paragraph, or delimited

# following the configuration, if any
{
   if nqp::defined($<configuration>) {
       :my $c := $<configuration>;
   
       if $c ~~ / 'V<#>' / {
           $*USE-HASH-MARK := 0:
       }
       # other tests following...
   }
}

[
  <?{ $*USE-HASH-MARK }> # if true, the rest of the token is evaluated for a match
                         # if false, the token will not match at all
  
  \s* <numbered-alias> \s+ <?before \w>
  
]?