An error correcting parser for context free grammars that takes less than cubic time

The problem of parsing has been studied extensively for various formal grammars. Given an input string and a grammar, the parsing problem is to check if the input string belongs to the language generated by the grammar. A closely related problem of great importance is one where the input are a string cal I and a grammar G and the task is to produce a string cal I' that belongs to the language generated by G and the `distance' between cal I and cal I' is the smallest (from among all the strings in the language). Specifically, if cal I is in the language generated by G, then the output should be cal I. Any parser that solves this version of the problem is called an em error correcting parser. In 1972 Aho and Peterson presented a cubic time error correcting parser for context free grammars. Since then this asymptotic time bound has not been improved under the (standard) assumption that the grammar size is a constant. In this paper we present an error correcting parser for context free grammars that runs in O(T(n)) time, where n is the length of the input string and T(n) is the time needed to compute the tropical product of two n× n matrices. In this paper we also present an fracnM-approximation algorithm for the em language edit distance problem that has a run time of O(Mnω), where O(nω) is the time taken to multiply two n× n matrices. To the best of our knowledge, no approximation algorithms have been proposed for error correcting parsing for general context free grammars.