Course of Raku / Regexes and grammars / Grammars / The parse tree, make and made

The match tree

A grammar match is a tree. The top match has a named entry for each token that TOP used, and those entries are themselves match objects with their own captures. You navigate the tree with the same <name> syntax you use for named captures:

grammar Pair {
    token TOP   { <key> '=' <value> }
    token key   { \w+ }
    token value { \d+ }
}

my $m = Pair.parse('x=5');
say $m<key>;   # ｢x｣
say $m<value>; # ｢5｣

Each branch is a full match object, so you can ask it for its .Str, its position, or convert it. Here the value is digits, so turning it into a real number is just a method call:

say $m<value>.Int; # 5

For deeper grammars the tree has more levels — $m<a><b> reaches a token b used inside a token a. Walking the tree like this works, but for anything beyond a couple of fields it becomes clumsy. The next topic shows a cleaner way to attach the value you actually want to each match.

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