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Instead of specifying an entire tree grammar, a tree pattern matcher lets us focus on just those subtrees we care
about. That's useful because different phases of a language application care about different parts of the tree. A tree pattern matcher also decouples the order in which we apply tree patterns from the tree patterns themselves. Unlike embedded walkers, visitors, and tree grammars, tree patterns don't specify how to walk the tree. ANTLR's new tree pattern matching engine dictates the tree traversal strategy. In its simplest form, a pattern matcher repeatedly tries to match patterns against subtrees. When it finds a match, the pattern matcher triggers an action or tree rewrite.
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