Recently I have been trying to create in Haskell a regex interpretor. What I did was create a new data type with all possible constructors (for sequence, *, ^, intervals, etc) and then define a matcher function. It works wonders but my problem is that I have to convert the input (the String, for example "a(b*)(c|d)ef") to my data type ("Seq (Sym a) (Seq (Rep Sym b) (Seq (Or Sym c Sym d) Sym ef))"). I am having trouble with this part of the problem (I tried creating a new data type, a parsing tree, but I failed completely). Any ideas on how I could solve it?
4
votes
In case you're not building this just for fun, there is also Text.Regex
– Jani Hartikainen
haskell.org/haskellwiki/Parsec: I don't know its details, but it's a really good library for parsing... playing with it also teaches you many things about monads.
– Riccardo T.
2 Answers
8
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The canonical approach is to use a parser combinator library, such as Parsec. Parser combinator libraries (like parser generators) let you write descriptions of your grammar, yielding a parser from strings to tokens in that language.
You simply have to encode your grammar as a Parsec function.
As an example, see this previous SO question: Using Parsec to parse regular expressions
