rex-0.2: A quasi-quoter for typeful results of regex captures.

MaintainerMichael Sloan (



This module provides a template Haskell quasiquoter for regular expressions, which provides the following features:

1) Compile-time checking that the regular expression is valid.

2) Arity of resulting tuple based on the number of selected capture patterns in the regular expression.

3) Allows for the inline interpolation of mapping functions :: String -> a.

4) Precompiles the regular expression at compile time, by calling into the PCRE library and storing a ByteString literal representation of its state.

5) Compile-time configurable to use different PCRE options, turn off precompilation, or use ByteStrings.

Since this is a quasiquoter library that generates code using view patterns, the following extensions are required:

 {-# LANGUAGE TemplateHaskell, QuasiQuotes, ViewPatterns #-}

Here's a silly example which parses peano numbers of the form Z, S Z, S S Z, etc. The s+ means that it is not sensitive to the quantity or type of seperating whitespace. (these examples can also be found in Test.hs)

 peano :: String -> Maybe Int
 peano = [rex|^(?{ length . filter (=='S') } \s* (?:S\s+)*Z)\s*$|]
 *Main> peano "Z"
 Just 0
 *Main> peano "S Z"
 Just 1
 *Main> peano "S   S Z"
 Just 2
 *Main> peano "S S S Z"
 Just 3
 *Main> peano "invalid"

The token "(?{" introduces a capture group which has a mapping applied to the -- result - in this case "length . filter (==S)". If the ?{ ... } are omitted, then the capture group is not taken as part of the results of the match. If the contents of the ?{ ... } is omitted, then id is assumed:

parsePair :: String -> Maybe (String, String) parsePair = [rex|^\s* (?{ }[^\s,]+) s*,s* (?{ }[^s,>]+) s*>$|]

The following example is derived from

 parseDate :: String -> Maybe (Int, Int, Int)
 parseDate [rex|^(?{ read -> y }(?:19|20)\d\d)[- /.]
                 (?{ read -> m }0[1-9]|1[012])[- /.]
                 (?{ read -> d }0[1-9]|[12][0-9]|3[01])$|]
   |  (d > 30 && (m `elem` [4, 6, 9, 11]))
   || (m == 2 &&
       (d == 29 && not (mod y 4 == 0 && (mod y 100 /= 0 || mod y 400 == 0)))
      || (d > 29)) = Nothing
   | otherwise = Just (y, m, d)
 parseDate _ = Nothing

The above example makes use of the regex quasi-quoter as a pattern matcher. The interpolated Haskell patterns are used to construct an implicit view pattern out of the inlined ones. The above pattern is expanded to the equivalent:

 parseDate ([rex|^(?{ read }(?:19|20)\d\d)[- /.]
                  (?{ read }0[1-9]|1[012])[- /.]
                  (?{ read }0[1-9]|[12][0-9]|3[01])$|]
           -> Just (y, m, d))

Caveat: Since haskell-src-exts does not support parsing view-patterns, the above is implemented as a relatively naive split on "->". It presumes that the last "->" in the interpolated pattern seperates the pattern from an expression on the left. This allows for lambdas to be present in the expression, but prevents nesting view patterns.

There are also a few other inelegances:

1) PCRE captures, unlike .NET regular expressions, yield the last capture made by a particular pattern. So, for example, (...)*, will only yield one match for .... Ideally these would be detected and yield an implicit [a].

2) Patterns with disjunction between captures ((?{f}a) | (?{g}b)) will provide the empty string to one of f / g. In the case of pattern expressions, it would be convenient to be able to map multiple captures into a single variable / pattern, preferring the first non-empty option. The general logic for this is a bit complicated, and postponed for a later release.

3) While this error is believed to no longer exist, the following error

  <interactive>: out of memory (requested 17584491593728 bytes)

Used to occur when evaluating in GHCi, due to a bug in the way precompilation worked. If this happens, please report it, and as a temporary work around, make your own quasiquoter using "rexConf _ False _ _" to disable pre-compilation.

Since pcre-light is a wrapper over a C API, the most efficient interface is ByteStrings, as it does not natively speak Haskell lists. The [rex| ... ] quasiquoter implicitely packs the input into a bystestring, and unpacks the results to strings before providing them to your mappers. The "brex" QuasiQuoter is provided for this purpose. You can also define your own QuasiQuoter - the definitions of the default configurations are as follows:

 rex  = rexConf False True rexPCREOpts rexPCREExecOpts
 brex = rexConf True  True rexPCREOpts rexPCREExecOpts

As mentioned, the other Bool determines whether precompilation is used. Due to GHC staging restrictions, your configuration will need to be in a different module than its usage.

Inspired by / copy-modified from Matt Morrow's regexqq package:

And code from Erik Charlebois's interpolatedstring-qq package:



rex, brex :: QuasiQuoterSource

Default regular expression quasiquoter for Strings, and ByteStrings.

maybeRead :: Read a => String -> Maybe aSource

A possibly useful utility function - yields Just x when there is a valid parse, and Nothing otherwise.

padRight :: a -> Int -> [a] -> [a]Source

Given a desired list-length, if the passed list is too short, it is padded with the given element. Otherwise, it trims.

rexConf :: Bool -> Bool -> [PCREOption] -> [PCREExecOption] -> QuasiQuoterSource

A configureable regular-expression QuasiQuoter. Takes the options to pass to the PCRE engine, along with Bools to flag ByteString usage and non-compilation respecively.