pcre2

Regular expressions for Haskell.

Teasers

licensePlate :: Text -> Maybe Text
licensePlate = match "[A-Z]{3}[0-9]{3,4}"

licensePlates :: Text -> [Text]
licensePlates = match "[A-Z]{3}[0-9]{3,4}"

case "The quick brown fox" of
    [regex|\bbrown\s+(?<animal>[A-z]+)\b|] -> Text.putStrLn animal
    _                                      -> error "nothing brown"

let kv'd = lined . packed . [_regex|(?x)  # Extended PCRE2 syntax
        ^\s*          # Ignore leading whitespace
        ([^=:\s].*?)  # Capture the non-empty key
        \s*           # Ignore trailing whitespace
        [=:]          # Separator
        \s*           # Ignore leading whitespace
        (.*?)         # Capture the possibly-empty value
        \s*$          # Ignore trailing whitespace
    |]

forMOf kv'd file $ execStateT $ do
    k <- gets $ capture @1
    v <- gets $ capture @2
    liftIO $ Text.putStrLn $ "found " <> k <> " set to " <> v

    case myMap ^. at k of
        Just v' | v /= v' -> do
            liftIO $ Text.putStrLn $ "setting " <> k <> " to " <> v'
            _capture @2 .= v'
        _ -> liftIO $ Text.putStrLn "no change"

Features

• No opaque "Regex" object. Instead, quiet functions with simple types—for the most part it's Text (pattern) -> Text (subject) -> result. Use partial application to create performant, compile-once-match-many code.
• No custom typeclasses.
• A single datatype for both compile and match options, the Option monoid.
• Text everywhere. See below for a version guide.
• Match success expressed via Alternative.
• Opt-in Template Haskell facilities for compile-time verification of patterns, indexing captures, and memoizing inline regexes.
• Opt-in lens support.
• No failure monads to express compile errors, preferring pure functions and throwing imprecise exceptions with pretty Show instances. Write simple code and debug it. Or, don't, and use the Template Haskell features instead. Both are first-class.
• Vast presentation of PCRE2 functionality. We can even register Haskell callbacks to run during matching!
• Zero-copying of substrings where beneficial.
• Few dependencies.
• Bundled, statically-linked UTF-8 (formerly UTF-16) build of up-to-date PCRE2 (version 10.44), with a complete, exposed Haskell binding.

Performance

Currently we are slower than other libraries. For example:

Operation	pcre2	pcre-light	regex-pcre-builtin
Compile and match a regex	3.9 μs	1.2 μs	2.9 μs

If it's really regex processing that's causing a bottleneck, pcre-light/-heavy/lens-regex-pcre are recommended instead of this library for the very best performance.

Unicode

Encoding	text version	pcre2 version	Code unit representation
UTF-8	≥ 2	≥ 2.2	Foreign.C.Types.CUChar
UTF-16	< 2	< 2.2	Foreign.C.Types.CUShort

Wishlist

• Many performance optimizations.
• Make use of DFA matching and JIT compilation.
• Improve PCRE2 C compile time.
• Add splitting support.

License

Apache 2.0.
PCRE2 is distributed under the 3-clause BSD license.

Main Author

©2020–2022 Shlomo Shuck