re2/README.md

# RE2, a regular expression library

RE2 is an efficient, principled regular expression library
that has been used in production at Google and many other places
since 2006.

_**Safety is RE2's primary goal.**_

RE2 was designed and implemented with an explicit goal of being able
to handle regular expressions from untrusted users without risk.
One of its primary guarantees is that the match time is linear in the
length of the input string. It was also written with production concerns in mind:
the parser, the compiler and the execution engines limit their memory usage
by working within a configurable budget—failing gracefully when exhausted—and
they avoid stack overflow by eschewing recursion.

It is not a goal to be faster than all other engines under all circumstances.
Although RE2 guarantees a running time that is asymptotically linear in
the length of the input, more complex expressions may incur larger constant factors;
longer expressions increase the overhead required to handle those expressions safely.
In a sense, RE2 is pessimistic where a backtracking engine is optimistic:
A backtracking engine tests each alternative sequentially, making it fast when the first alternative is common.
By contrast RE2 evaluates all alternatives in parallel, avoiding the performance penalty for the last alternative,
at the cost of some overhead. This pessimism is what makes RE2 secure.

It is also not a goal to implement all of the features offered by Perl, PCRE and other engines.
As a matter of principle, RE2 does not support constructs for which only backtracking solutions are known to exist.
Thus, backreferences and look-around assertions are not supported.

For more information, please refer to Russ Cox's articles on regular expression theory and practice:

* [Regular Expression Matching Can Be Simple And Fast](https://swtch.com/~rsc/regexp/regexp1.html)
* [Regular Expression Matching: the Virtual Machine Approach](https://swtch.com/~rsc/regexp/regexp2.html)
* [Regular Expression Matching in the Wild](https://swtch.com/~rsc/regexp/regexp3.html)

### Syntax

In POSIX mode, RE2 accepts standard POSIX (egrep) syntax regular expressions.
In Perl mode, RE2 accepts most Perl operators.  The only excluded ones are
those that require backtracking (and its potential for exponential runtime)
to implement.  These include backreferences (submatching is still okay)
and generalized assertions.
The [Syntax wiki page](https://github.com/google/re2/wiki/Syntax)
documents the supported Perl-mode syntax in detail.
The default is Perl mode.

### C++ API

RE2's native language is C++, although there are [ports and wrappers](#ports-and-wrappers) listed below.

#### Matching Interface

There are two basic operators:
`RE2::FullMatch` requires the regexp to match the entire input text, and
`RE2::PartialMatch` looks for a match for a substring of the input text,
returning the leftmost-longest match in POSIX mode and the
same match that Perl would have chosen in Perl mode.

Examples:

```cpp
assert(RE2::FullMatch("hello", "h.*o"))
assert(!RE2::FullMatch("hello", "e"))

assert(RE2::PartialMatch("hello", "h.*o"))
assert(RE2::PartialMatch("hello", "e"))
```

#### Submatch Extraction

Both matching functions take additional arguments in which submatches will be stored.
The argument can be a `string*`, or an integer type, or the type `absl::string_view*`.
(The `absl::string_view` type is very similar to the `std::string_view` type,
but for historical reasons, RE2 uses the former.)
A `string_view` is a pointer to the original input text, along with a count.
It behaves like a string but doesn't carry its own storage.
Like when using a pointer, when using a `string_view`
you must be careful not to use it once the original text has been deleted or gone out of scope.

Examples:

```cpp
// Successful parsing.
int i;
string s;
assert(RE2::FullMatch("ruby:1234", "(\\w+):(\\d+)", &s, &i));
assert(s == "ruby");
assert(i == 1234);

// Fails: "ruby" cannot be parsed as an integer.
assert(!RE2::FullMatch("ruby", "(.+)", &i));

// Success; does not extract the number.
assert(RE2::FullMatch("ruby:1234", "(\\w+):(\\d+)", &s));

// Success; skips NULL argument.
assert(RE2::FullMatch("ruby:1234", "(\\w+):(\\d+)", (void*)NULL, &i));

// Fails: integer overflow keeps value from being stored in i.
assert(!RE2::FullMatch("ruby:123456789123", "(\\w+):(\\d+)", &s, &i));
```

#### Pre-Compiled Regular Expressions

The examples above all recompile the regular expression on each call.
Instead, you can compile it once to an RE2 object and reuse that object for each call.

Example:
```cpp
RE2 re("(\\w+):(\\d+)");
assert(re.ok());  // compiled; if not, see re.error();

assert(RE2::FullMatch("ruby:1234", re, &s, &i));
assert(RE2::FullMatch("ruby:1234", re, &s));
assert(RE2::FullMatch("ruby:1234", re, (void*)NULL, &i));
assert(!RE2::FullMatch("ruby:123456789123", re, &s, &i));
```

#### Options

The constructor takes an optional second argument that can
be used to change RE2's default options.
For example, `RE2::Quiet` silences the error messages that are
usually printed when a regular expression fails to parse:

```cpp
RE2 re("(ab", RE2::Quiet);  // don't write to stderr for parser failure
assert(!re.ok());  // can check re.error() for details
```

Other useful predefined options are `Latin1` (disable UTF-8) and `POSIX`
(use POSIX syntax and leftmost longest matching).

You can also declare your own `RE2::Options` object and then configure it as you like.
See the [header](https://github.com/google/re2/blob/main/re2/re2.h) for the full set of options.

#### Unicode Normalization

RE2 operates on Unicode code points: it makes no attempt at normalization.
For example, the regular expression /ü/ (U+00FC, u with diaeresis)
does not match the input "ü" (U+0075 U+0308, u followed by combining diaeresis).
Normalization is a long, involved topic.
The simplest solution, if you need such matches, is to normalize both the regular expressions
and the input in a preprocessing step before using RE2.
For more details on the general topic, see <https://www.unicode.org/reports/tr15/>.

#### Additional Tips and Tricks

For advanced usage, like constructing your own argument lists,
or using RE2 as a lexer, or parsing hex, octal, and C-radix numbers,
see [re2.h](https://github.com/google/re2/blob/main/re2/re2.h).

### Installation

RE2 can be built and installed using GNU make, CMake, or Bazel.
The simplest installation instructions are:

	make
	make test
	make benchmark
	make install
	make testinstall

Building RE2 requires a C++17 compiler and the [Abseil](https://github.com/abseil/abseil-cpp) library.
Building the tests and benchmarks requires
[GoogleTest](https://github.com/google/googletest)
and [Benchmark](https://github.com/google/benchmark).
To obtain those:

- Linux: `apt install libabsl-dev libgtest-dev libbenchmark-dev`
- macOS: `brew install abseil googletest google-benchmark pkg-config-wrapper`
- Windows: `vcpkg install abseil gtest benchmark` \
  or `vcpkg add port abseil gtest benchmark`

Once those are installed, the build has to be able to find them.
If the standard Makefile has trouble, then switching to CMake can help:

	rm -rf build
	cmake -DRE2_TEST=ON -DRE2_BENCHMARK=ON -S . -B build
	cd build
	make
	make test
	make install

When using CMake, with benchmarks enabled, `make test` builds and runs test binaries
and builds a `regexp_benchmark` binary but does not run it.
If you don't need the tests or benchmarks at all, you can omit the corresponding `-D` arguments,
and then you don't need the GoogleTest or Benchmark dependencies either.

Another useful option is `-DRE2_USE_ICU=ON`, which adds a dependency on the
ICU Unicode library but also extends the list of property names available in the `\p` and `\P` patterns.

CMake can also be used to generate Visual Studio and Xcode projects, as well as
Cygwin, MinGW, and MSYS makefiles.

 - Visual Studio users: You need Visual Studio 2019 or later.
 - Cygwin users: You must run CMake from the Cygwin command line, not the Windows command line.

If you are adding RE2 to your own CMake project,
CMake has two ways to use a dependency: `add_subdirectory()`,
which is when the dependency's **_sources_** are in a subdirectory of your project;
and `find_package()`, which is when the dependency's
**_binaries_** have been built and installed somewhere on your system.
The Abseil documentation walks through the former [here](https://abseil.io/docs/cpp/quickstart-cmake)
versus the latter [here](https://abseil.io/docs/cpp/tools/cmake-installs).
Once you get Abseil working, getting RE2 working will be a very similar process and,
either way, `target_link_libraries(… re2::re2)` should Just Work™.

If you are using [Bazel](https://bazel.io), it will handle the dependencies for you,
although you still need to download Bazel,
which you can do with [Bazelisk](https://github.com/bazelbuild/bazelisk).

	go install github.com/bazelbuild/bazelisk@latest
	# or on mac: brew install bazelisk

	bazelisk build :all
	bazelisk test :all

If you are using RE2 from another project, you need to make sure you are
using at least C++17.
See the RE2 [.bazelrc](https://github.com/google/re2/blob/main/.bazelrc) file for an example.

### Ports and Wrappers

RE2 is implemented in C++.

The official Python wrapper is [in the `python` directory](https://github.com/google/re2/tree/main/python)
and [published on PyPI as `google-re2`](https://pypi.org/project/google-re2/).
Note that there is also a PyPI `re2` but it is not by the RE2 authors and is unmaintained. Use `google-re2`.

There are also other unofficial wrappers:

- A C wrapper is at <https://github.com/marcomaggi/cre2/>.
- A D wrapper is at <https://github.com/ShigekiKarita/re2d/> and [on DUB](https://code.dlang.org/packages/re2d).
- An Erlang wrapper is at <https://github.com/dukesoferl/re2/> and [on Hex](https://hex.pm/packages/re2).
- An Inferno wrapper is at <https://github.com/powerman/inferno-re2/>.
- A Node.js wrapper is at <https://github.com/uhop/node-re2/> and [on NPM](https://www.npmjs.com/package/re2).
- An OCaml wrapper is at <https://github.com/janestreet/re2/> and [on OPAM](https://opam.ocaml.org/packages/re2/).
- A Perl wrapper is at <https://github.com/dgl/re-engine-RE2/> and [on CPAN](https://metacpan.org/pod/re::engine::RE2).
- An R wrapper is at <https://github.com/girishji/re2/> and [on CRAN](https://cran.r-project.org/web/packages/re2/index.html).
- A Ruby wrapper is at <https://github.com/mudge/re2/> and on RubyGems (rubygems.org).
- A WebAssembly wrapper is at <https://github.com/google/re2-wasm/> and on NPM (npmjs.com).

[RE2J](https://github.com/google/re2j) is a port of the RE2 C++ code to pure Java,
and [RE2JS](https://github.com/le0pard/re2js) is a port of RE2J to JavaScript.

The [Go `regexp` package](https://go.dev/pkg/regexp)
and [Rust `regex` crate](https://docs.rs/regex)
do not share code with RE2, but they follow the same principles,
accept the same syntax, and provide the same efficiency guarantees.

### Contact

The [issue tracker](https://github.com/google/re2/issues) is the best place for discussions.

There is a [mailing list](https://groups.google.com/group/re2-dev) for keeping up with code changes.

Please read the [contribution guide](https://github.com/google/re2/wiki/Contribute) before sending changes.
In particular, note that RE2 does not use GitHub pull requests.