Editing Parsing expression grammar (section)

=== Semantics ===

The fundamental difference between [[context-free grammars]] and parsing expression grammars is that the PEG's choice operator is ''ordered''. If the first alternative succeeds, the second alternative is ignored. Thus ordered choice is not [[commutativity|commutative]], unlike unordered choice as in context-free grammars. Ordered choice is analogous to [[cut (logic programming)|soft cut]] operators available in some [[logic programming]] languages.

The consequence is that if a CFG is transliterated directly to a PEG, any ambiguity in the former is resolved by deterministically picking one parse tree from the possible parses. By carefully choosing the order in which the grammar alternatives are specified, a programmer has a great deal of control over which parse tree is selected.

Parsing expression grammars also add the and- and not- [[syntactic predicate]]s. Because they can use an arbitrarily complex sub-expression to "look ahead" into the input string without actually consuming it, they provide a powerful syntactic [[Parsing#Lookahead|lookahead]] and disambiguation facility, in particular when reordering the alternatives cannot specify the exact parse tree desired.