option

Given a subrule 'r', represents the expression 'r?'. It tries to match 'r' and if this matches successfully, it returns this match. If 'r' failed, 'r?' is still a success, but without any child nor match.

alias option!(literal!"abc") rule; // Aka '"abc"?'
ParseTree input = ParseTree("",false,[],"abcd");

ParseTree result = rule(input);
assert(result.successful);
assert(result.matches == ["abc"]);
assert(result.children.length == 1);
assert(result.children[0] == literal!"abc"(input));