[4] is not really a usable 'product'. Livegrep (https://github.com/livegrep/livegrep https://github.com/livegrep/livegrep) was inspired by it and is very usable

[4] is not really a usable 'product'. Livegrep (https://github.com/livegrep/livegrep https://github.com/livegrep/livegrep) was inspired by it and is very usable. [3] used to be a Google open source project as well, but it fell out of maintenance, and Sourcegraph took it over. It powers most of the basic regex/literal search in Sourcegraph. Mozilla's code is searchable in Searchfox (https://searchfox.org/ https://searchfox.org/) which uses the indexer from Livegrep, combined with their own Git indexer and language-specific cross reference databases. OpenGrok (https://github.com/oracle/opengrok https://github.com/oracle/opengrok) is also rather well known, but I have found it to have a slightly worse UI than alternatives.