pcre2-2.3.1: README.md
# pcre2
[](https://hackage.haskell.org/package/pcre2)
Regular expressions for Haskell.
## Teasers
```haskell
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
```
```haskell
case "The quick brown fox" of
[regex|\bbrown\s+(?<animal>[A-z]+)\b|] -> Text.putStrLn animal
_ -> die "nothing brown"
```
```haskell
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](https://hackage.haskell.org/package/regex-base/docs/Text-Regex-Base-RegexLike.html#t:RegexContext).
* 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](https://hackage.haskell.org/package/pcre-light)/[-heavy](https://hackage.haskell.org/package/pcre-heavy)/[lens-regex-pcre](https://hackage.haskell.org/package/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](https://www.apache.org/licenses/LICENSE-2.0.html).
PCRE2 is distributed under the [3-clause BSD](https://www.pcre.org/licence.txt) license.
## Main Author
©2020–2026 Steven Shuck