In computer science, the Earley parser is an algorithm for parsing strings that belong to a given context-free language, though (depending on the variant) it may suffer problems with certain nullable grammars.

[1] The algorithm, named after its inventor, Jay Earley, is a chart parser that uses dynamic programming; it is mainly used for parsing in computational linguistics.

The Earley parser executes in cubic time in the general case

Each state is a tuple (X → α • β, i), consisting of (Earley's original algorithm included a look-ahead in the state; later research showed this to have little practical effect on the parsing efficiency, and it has subsequently been dropped from most implementations.)

Adapted from Speech and Language Processing[5] by Daniel Jurafsky and James H. Martin, Consider the following simple grammar for arithmetic expressions: With the input: This is the sequence of state sets: The state (P → S •, 0) represents a completed parse.

Earley's dissertation[6] briefly describes an algorithm for constructing parse trees by adding a set of pointers from each non-terminal in an Earley item back to the items that caused it to be recognized.

But Tomita noticed[7] that this does not take into account the relations between symbols, so if we consider the grammar S → SS | b and the string bbb, it only notes that each S can match one or two b's, and thus produces spurious derivations for bb and bbbb as well as the two correct derivations for bbb.

Another method[8] is to build the parse forest as you go, augmenting each Earley item with a pointer to a shared packed parse forest (SPPF) node labelled with a triple (s, i, j) where s is a symbol or an LR(0) item (production rule with dot), and i and j give the section of the input string derived by this node.

Philippe McLean and R. Nigel Horspool in their paper "A Faster Earley Parser" combine Earley parsing with LR parsing and achieve an improvement in an order of magnitude.