Google Code Search
|Initial release||October 5, 2006|
|Operating system||Any (web-based application)|
|Type||Code search engine|
Google Code Search was a free beta product from Google which debuted in Google Labs on October 5, 2006, allowing web users to search for open-source code on the Internet. Features included the ability to search using operators, namely lang:, package:, license: and file:.
Regular expression engine
The site allowed the use of regular expressions in queries, which at the time was not offered by any other search engine for code. This makes it resemble grep, but over the world's public code. The methodology employed combines a trigram index with a custom-built, denial-of-service resistant regular expression engine.
Google Code Search supported POSIX extended regular expression syntax, excluding back-references, collating elements, and collation classes.
Languages not officially supported could be searched for using the file: operator to match the common file extensions for the language.
In January 2012, Russ Cox published an overview of history and the technical aspects of the tool, and open-sourced a basic implementation of a similar functionality as a set of standalone programs that can run fast indexed regular expression searches over local code.
- Sourcegraph 
- Open Hub (a merge of Ohloh and Koders)
- SearchCode 
- Russ Cox (January 2012). "Regular Expression Matching with a Trigram Index (or: How Google Code Search Worked)".
- Google Open Source Blog: RE2: a principled approach to regular expression matching
- Horowitz, Bradley (2011-10-14). "Official Blog: A fall sweep". Googleblog.blogspot.com. Retrieved 2013-07-09.
- codesearch on GitHub