This confused me when I was learning Haskell as well. The way I understand it is that regex-base defines a base-line interface -- such as the context-aware =~ function -- that all of the other packages implement, which is why they're known as 'backends' (e.g. regex-tdfa, regex-pcre, etc.).

In newer versions of GHC (> 6.6), the backends re-export this interface so that you just need to import the backend. regex-tdfa is the backend that I've used myself, mainly because I saw it used in other packages (Hakyll).

http://www.haskell.org/haskellwiki/Regular_expressions#regex...

Text.Regex is provided by regex-comp. (Better install haskell platform).