pcre2

Regular expressions for Haskell.

Teasers

licensePlateRegex :: (Alternative f) => Text -> f Text
licensePlateRegex = match "[A-Z]{3}[0-9]{3,4}"

firstLicensePlate :: Text -> Maybe Text
firstLicensePlate = licensePlateRegex

allLicensePlates :: Text -> [Text]
allLicensePlates = licensePlateRegex

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

let kv'd :: Traversal' String (Captures _)
    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
    |]

forOf kv'd file $ \kv -> do
    let k = capture @1 kv
        v = capture @2 kv

    Text.putStrLn $ "found " <> k <> " set to " <> v

    kv' <- case Map.lookup k replacements of
        Just v' | v /= v' -> do
            Text.putStrLn $ "setting " <> k <> " to " <> v'
            return $ set (_capture @2) v' kv
        _ -> do
            Text.putStrLn $ "no change for " <> k
            return kv

    ...

Features

• No opaque "Regex" object. Instead, quiet functions with simple types—for the most part it's Text (pattern) -> Text (subject) -> result.
• No custom typeclasses.
• A single datatype for both compile and match options, the Option monoid.
• 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 build of up-to-date PCRE2 (version 10.47), with a complete, exposed Haskell binding.

Performance

Currently (2026-05-02) we are much slower than other libraries.

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.

Wishlist

• Many performance optimizations.
• Make use of DFA matching for lazy (infinite) inputs. This likely requires some upstream changes as well but in theory it's possible.
• Improve compile time.
  • Support external libpcre2 maybe.
• Support more types than just Text.

License

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

Main Author

©2020–2026 Steven Shuck