Bug reports, suggestions and contributions are welcome.
The following styles of regular expressions are supported:
- Perl-style regular expressions (module
- Posix extended regular expressions (module
- Emacs-style regular expressions (module
- Shell-style file globbing (module
It is also possible to build regular expressions by combining simpler regular
The most notable missing features are back-references and look-ahead/look-behind assertions.
There is also a subset of the PCRE interface available in the
This makes it easier to port code from that library to Re minimal changes.
The matches are performed by lazily building a DFA (deterministic finite automaton) from the regular expression. As a consequence, matching takes linear time in the length of the matched string.
The compilation of patterns is slower than with libraries using back-tracking, such as PCRE. But, once a large enough part of the DFA is built, matching is extremely fast.
Of course, for some combinations of regular expression and string, the part of the DFA that needs to be build is so large that this point is never reached, and matching will be slow. This is not expected to happen often in practice, and actually a lot of expressions that behaves badly with a backtracking implementation are very efficient with this implementation.
The library is at the moment entirely written in OCaml. As a consequence, regular expression matching is much slower when the library is compiled to bytecode than when it is compiled to native code.
Here are some timing results (Pentium III 500Mhz):
Scanning a 1Mb string containing only
as, except for the last character which is a
b, searching for the pattern
aa?b(repeated 100 times):
- RE: 2.6s
- PCRE: 68s
Regular expression example from http://www.bagley.org/~doug/shootout/ 
- RE: 0.43s
- PCRE: 3.68s
 this page is no longer up but is available via the Internet Archive http://web.archive.org/web/20010429190941/http://www.bagley.org/~doug/shootout/bench/regexmatch/
The large regular expression (about 2000 characters long) that Unison uses with my preference file to decide whether a file should be ignored or not. This expression is matched against a filename about 20000 times.
- RE: 0.31s
- PCRE: 3.7s However, RE is only faster than PCRE when there are more than about 300 filenames.