Regular expressions in Haskell

A Regular expression (regexp), such as ``a+b+'', is a pattern that can be used to express a searching problem. (In this case the pattern matches a string of 1 or more a's followed by a string of 1 or more b's.)

There are a number of regular expression libraries around. One of the best can be found in the Perl programming language. The new GHC regexp library, provided here, is a regular expression package for the purely functional language Haskell. It has a number of important features:

It is written purely in Haskell.

It provides polymorphic regular expressions - instead of just searching along strings, we can search along a list of any type over which equality is defined.
For example, given a temperature type,
```
data Temp   = Celsius Int 
            | Fahrenheit Int
            | Kelvin Int
```
and a list of Temp elements,
```
   
    xs = [Kelvin 200, Kelvin 200, Celcius 5]
```
we could use the polymorphic library to search for a sequence of Kelvin 200:
```
    search "< Kelvin 200>*" xs
```
It provides a specific string regular expression syntax to allow all the best features of Perl regexps - eg \w to match any alphanumeric.
It has user defined matcher functions - with these we can match balanced text. eg match an equal number of opening and closing parentheses eg (((...)))
It provides a powerful, easy to use syntax by equating regexp matching with Haskell pattern matching.
For example, we could write:
```
case prog of 
     </--.*\n/> -> A -- deal with comment
     </>.*\n/> -> B  -- deal with line of code
     _ -> C          -- otherwise
```
This would do A if a comment (line beginning with --) was found. It would do B, if a line of code (line beginning with >) were found. Otherwise it would do C.

The regexp library works with all current versions of GHC (the Glasgow haskell compiler).

The regexp library (including all documentation) is available in three versions.

For Haskell 1.2 (GHC up to ghc-0.29) here.

For Haskell 1.3/1.4 (ghc-2.01 onwards, and hugs-1.3 or hugs-1.4 Yale Release) here.

For Haskell 98 here.
Author
Meurig Sage,
Department of Computing Science,
University of Glasgow,
Glasgow G12 8QQ,
Scotland
UK
Email: meurig@dcs.gla.ac.uk

Regular expressions in Haskell

Author