haskell regex substitution
Despite the ridiculously large number of regex matching engines开发者_Python百科 for Haskell, the only one I can find that will substitute is Text.Regex
, which, while decent, is missing a few thing I like from pcre. Are there any pcre-based packages which will do substitution, or am I stuck with this?
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
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
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.
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