haskell regex substitution

Update 2020

I totally agree with @rlpowell that

I don't think "just roll your own" is a reasonable answer to people trying to get actual work done, in an area where every other modern language has a trivial way to do this.

At the time of this writing, there is also Regex.Applicative.replace for regex substitution, though it's not Perl-compatible.

For pattern-matching and substitution with parsers instead of regex, there is Replace.Megaparsec.streamEdit

I don't think "just roll your own" is a reasonable answer to people trying to get actual work done, in an area where every other modern language has a trivial way to do this. Including Scheme. So here's some actual resources; my code is from a project where I was trying to replace "qql foo bar baz qq" with text based on calling a function on the stuff inside the qq "brackets", because reasons.

Best option: pcre-heavy:

      let newBody = gsub [re|\s(qq[a-z]+)\s(.*?)\sqq\s|] (unWikiReplacer2 titles) body in do
[snip]
unWikiReplacer2 :: [String] -> String -> [String] -> String
unWikiReplacer2 titles match subList = case length subList > 0 of
        True -> " --" ++ subList!!1 ++ "-- "
        False -> match

Note that pcre-heavy directly supports function-based replacement, with any string type. So nice.

Another option: pcre-light with a small function that works but isn't exactly performant:

    let newBody = replaceAllPCRE "\\s(qq[a-z]+)\\s(.*?)\\sqq\\s" (unWikiReplacer titles) body in do
[snip]
unWikiReplacer :: [String] -> (PCRE.MatchResult String) -> String
unWikiReplacer titles mr = case length subList > 0 of
        True -> " --" ++ subList!!1 ++ "-- "
        False -> PCRE.mrMatch mr
    where
        subList = PCRE.mrSubList mr

-- A very simple, very dumb "replace all instances of this regex
-- with the results of this function" function.  Relies on the
-- MatchResult return type.
--
-- https://github.com/erantapaa/haskell-regexp-examples/blob/master/RegexExamples.hs
-- was very helpful to me in constructing this
--
-- I also used
-- https://github.com/jaspervdj/hakyll/blob/ea7d97498275a23fbda06e168904ee261f29594e/src/Hakyll/Core/Util/String.hs
replaceAllPCRE :: String              -- ^ Pattern
           -> ((PCRE.MatchResult String) -> String)  -- ^ Replacement (called on capture)
           -> String              -- ^ Source string
           -> String              -- ^ Result
replaceAllPCRE pattern f source =
    if (source PCRE.=~ pattern) == True then
      replaceAllPCRE pattern f newStr
    else
      source
    where
        mr = (source PCRE.=~ pattern)
        newStr = (PCRE.mrBefore mr) ++ (f mr) ++ (PCRE.mrAfter mr)

Someone else's fix: http://0xfe.blogspot.com/2010/09/regex-substitution-in-haskell.html

Another one, this time embedded in a major library: https://github.com/jaspervdj/hakyll/blob/master/src/Hakyll/Core/Util/String.hs

Another package for this purpose: https://hackage.haskell.org/package/pcre-utils

The regular expression API in regex-base is generic to the container of characters to match. Doing some kind of splicing generically to implements substitution would be very hard to make efficient. I did not want to provide a crappy generic routine.

Writing a small function to do the substitution exactly how you want is just a better idea, and it can be written to match your container.