Google Robots.txt Parser and Matcher Library

The repository contains Google's robots.txt parser and matcher as a C++ library (compliant to C++11).

About the library

The Robots Exclusion Protocol (REP) is a standard that enables website owners to control which URLs may be accessed by automated clients (i.e. crawlers) through a simple text file with a specific syntax. It's one of the basic building blocks of the internet as we know it and what allows search engines to operate.

Because the REP was only a de-facto standard for the past 25 years, different implementers implement parsing of robots.txt slightly differently, leading to confusion. This project aims to fix that by releasing the parser that Google uses.

The library is slightly modified (i.e. some internal headers and equivalent symbols) production code used by Googlebot, Google's crawler, to determine which URLs it may access based on rules provided by webmasters in robots.txt files. The library is released open-source to help developers build tools that better reflect Google's robots.txt parsing and matching.

For webmasters, we included a small binary in the project that allows testing a single URL and user-agent against a robots.txt.

Building the library

Bazel is the official build system for the library, which is supported on most major platforms (Linux, Windows, MacOS, for example) and compilers.

CMake support may be added in a future release. If you want to help us with adding CMake support, pull requests are highly welcome!

Quickstart

We included with the library a small binary to test a local robots.txt against a user-agent and URL. Running the included binary requires:

• A compatible platform (e.g. Windows, Mac OS X, Linux, etc.). Most platforms are fully supported.
• A compatible C++ compiler supporting at least C++11. Most major compilers are supported.
• Git for interacting with the source code repository. To install Git, consult the Set Up Git guide on GitHub.
• Although you are free to use your own build system, most of the documentation within this guide will assume you are using Bazel. To download and install Bazel (and any of its dependencies), consult the Bazel Installation Guide.

To build and run the binary:

$ git clone https://github.com/google/robotstxt.git robotstxt
Cloning into 'robotstxt'...
...
$ cd robotstxt/
bazel-robots$ bazel test :robots_test
...
/:robots_test                                                      PASSED in 0.1s

Executed 1 out of 1 test: 1 test passes.
...
bazel-robots$ bazel build :robots_main
...
Target //:robots_main up-to-date:
  bazel-bin/robots_main
...
bazel-robots$ bazel run robots_main -- ~/local/path/to/robots.txt YourBot http://example.com/url
  user-agent 'YourBot' with url 'http://example.com/url' allowed: YES

Notes

Parsing of robots.txt files themselves is done exactly as in the production version of Googlebot, including how percent codes and unicode characters in patterns are handled. The user must ensure however that the URI passed to the AllowedByRobots and OneAgentAllowedByRobots functions, or to the URI parameter of the robots tool, follows the format specified by RFC3986, since this library will not perform full normalization of those URI parameters. Only if the URI is in this format, the matching will be done according to the REP specification.

License

The robots.txt parser and matcher C++ library is licensed under the terms of the Apache license. See LICENSE for more information.

Links

To learn more about this project:

• check out the internet draft,
• how Google's handling robots.txt,
• or for a high level overview, the robots.txt page on Wikipedia.