agrep (approximate grep) is an open-source approximate string matching program, developed by Udi Manber and Sun Wu between 1988 and 1991,^[1] for use with the Unix operating system. It was later ported to OS/2, DOS, and Windows.

It selects the best-suited algorithm for the current query from a variety of the known fastest (built-in) string searching algorithms, including Manber and Wu's bitap algorithm based on Levenshtein distances.

agrep is also the search engine in the indexer program GLIMPSE. agrep is under a free ISC License.^[2]

Alternative implementations

A more recent agrep is the command-line tool provided with the TRE regular expression library. TRE agrep is more powerful than Wu-Manber agrep since it allows weights and total costs to be assigned separately to individual groups in the pattern. It can also handle Unicode.^[3] Unlike Wu-Manber agrep, TRE agrep is licensed under a 2-clause BSD-like license.

FREJ (Fuzzy Regular Expressions for Java) open-source library provides command-line interface which could be used in the way similar to agrep. Unlike agrep or TRE it could be used for constructing complex substitutions for matched text.^[4] However its syntax and matching abilities differs significantly from ones of ordinary regular expressions.