如何在 Haskell 中表述正则表达式模式？

Question

我正在尝试使用以下代码进行正则表达式替换

import Text.RE.Replace
import Text.RE.TDFA.String

onlyLetters :: String -> String
onlyLetters s = replaceAll "" $ s *=~ [re|$([^a-zA-Z])|]

我发现很难找到任何关于此的可理解的文档。 这会产生编译错误：

    src\Pangram.hs:6:53: error: parse error on input `]'
  |
6 | onlyLetters s = replaceAll "" $ (s *=~ [re|[a-zA-Z]|])
  |                                                     ^

Progress 1/2

--  While building package pangram-2.0.0.12 (scroll up to its section to see the error) using:
      C:\sr\setup-exe-cache\x86_64-windows\Cabal-simple_Z6RU0evB_3.0.1.0_ghc-8.8.4.exe --builddir=.stack-work\dist\29cc6475 build lib:pangram test:test --ghc-options " -fdiagnostics-color=always"
    Process exited with code: ExitFailure 1
PS C:\Users\mcleg\Exercism\haskell\pangram> stack test
pangram> configure (lib + test)
Configuring pangram-2.0.0.12...
pangram> build (lib + test)
Preprocessing library for pangram-2.0.0.12..
Building library for pangram-2.0.0.12..
[1 of 2] Compiling Pangram

src\Pangram.hs:7:56: error: parse error on input `]'
  |
7 | onlyLetters s = replaceAll "" $ s *=~ [re|$([^a-zA-Z])|]
  |                                                        ^

Progress 1/2

--  While building package pangram-2.0.0.12 (scroll up to its section to see the error) using:
      C:\sr\setup-exe-cache\x86_64-windows\Cabal-simple_Z6RU0evB_3.0.1.0_ghc-8.8.4.exe --builddir=.stack-work\dist\29cc6475 build lib:pangram test:test --ghc-options " -fdiagnostics-color=always"
    Process exited with code: ExitFailure 1

该支架有什么问题，我该如何正确执行此操作？ 谢谢 -斯凯

Answer 1

[…|…|]

是准引用语法^{[haskell-wiki]}。这是 Haskell 语法的扩展，默认情况下不启用。

您可以使用

LANGUAGE

编译指示打开此功能：

{-# LANGUAGE QuasiQuotes #-}

import Text.RE.Replace
import Text.RE.TDFA.String

onlyLetters :: String -> String
onlyLetters s = replaceAll "" $ s *=~ [re|$([^a-zA-Z])|]

准引号将生成 Haskell 代码，然后在 Haskell 程序中使用。这意味着通过准引号，可以在编译时完成正则表达式的验证，与在运行时编译正则表达式相比，甚至可能稍微优化效率。

对于给定的

onlyLetters

函数，我们得到：

*Main> onlyLetters "fo0b4r"
"fobr"

Answer 2

Willem Van Onsem 的答案是对这个问题的更好答案，但我会建议一个 “试试这个” 回答。

这就是如何在普通 Haskell 中进行文本替换，而无需使用准引用正则表达式的复杂性。

与 https://hackage.haskell.org/package/replace-megaparsec/docs/Replace-Megaparsec.html#v:streamEdit

{-# LANGUAGE TypeFamilies #-}

import Text.Megaparsec
import Text.Megaparsec.Char
import Replace.Megaparsec
import Data.Void

-- | Invert a single-token parser “character class”.
-- | For example, match any single token except a letter or whitespace: `anySingleExcept (letterChar <|> spaceChar)`
anySingleExcept :: (MonadParsec e s m, Token s ~ Char) => m (Token s) -> m (Token s)
anySingleExcept p = notFollowedBy p *> anySingle

-- | A parser monad pattern which matches anything except letters.
nonLetters :: Parsec Void String String
nonLetters = many (anySingleExcept letterChar) 

onlyLetters :: String -> String
onlyLetters = streamEdit nonLetters (const "")

onlyLetters "fo0b4r"

"fobr"