TRE is an open-source library for pattern matching in text, which works like a regular expression engine with the ability to do approximate string matching. It was developed by Ville Laurikari and is distributed under a 2-clause BSD-like license.

The library is written in C and provides functions which allow using regular expressions for searching over input text lines. The main difference from other regular expression engines is that TRE can match text fragments in an approximate way, that is, supposing that text could have some number of typos.

TRE uses extended regular expression syntax with the addition of "directions" for matching preceding fragment in approximate way. Each of such directions specifies how many typos are allowed for this fragment.

Approximate matching is performed in a way similar to Levenshtein distance, which means that there are three types of typos 'recognized':

TRE allows specifying of cost for each of three typos type independently.

The project comes with a command-line utility, a reimplementation of agrep.

Though approximate matching requires some syntax extension, when this feature is not used, TRE works like most of other regular expression matching engines. This means that

The library's author states that time spent for matching grows linearly with increasing of input text length, while memory requirement is constant during matching and does not depend on the input, only on the pattern.