Editing Regular expressions (section)

=== regex-base ===

This package exports Text.Regex.Base which re-exports Text.Regex.RegexLike and Text.Regex.Context.  These do not provide the ability to do matching, but provide the type classes which all the other regex-* backends use.

The backend packages also import the utility module Text.Regex.Impl to streamline instance declarations.

The 0.71 version has a "tail" bug in one of the instances of RegexLike:
<haskell>
instance (RegexLike a b) => RegexContext a b (MatchResult b) where 
</haskell>
which I hope to be fixed in ghc 6.6.1.  Getting the unstable version of regex-base also fixes this, though you will have to get and re-compile all the other regex-* modules as well.

The versions of the regex-* backends that come with GHC 6.6 do not re-export the RegexLike classes, so the usage of regex-BACKEND is
<haskell>
import Text.Regex.Base
import Text.Regex.BACKEND
</haskell>
The versions in unstable are being upgraded to re-export RegexLike, so the usage will be simplified to
<haskell>
import Text.Regex.BACKEND
</haskell>

The 0.71 version of regex-base only has Extract instances for [Char] and ByteString.  The unstable version also provides instances of ByteString.Lazy and (Seq Char).  This Extract support must be accompanied by adding support to each regex-* backend in unstable, and that work is in progress.

The RegexMaker class in v0.71 had no way to gracefully report errors in parsing the regular expression itself.  This was a design mistake and has been extended in the unstable version of regex-base to provide monadic version which can fail gracefully.

The RegexLike class only provides support for positions in the source text indexed by Int.  I am still considering how to provide Int64 support.  The best way is probably going to be another class, which will allow for more generalized index type support.  Different backends will, by necessity, have different instances for extended index types.