The Xapian project offers an Open Source Search Engine Library, available under the GPL v2+. Written in C++, with bindings for a variety of languages, Xapian is a versatile toolkit for developers to enhance indexing and search capabilities in their applications. With built-in support for different weighting models and boolean query operators, Xapian stands out for its flexibility. Omega, a search engine application built on Xapian, provides a packaged solution for website search, with the option to customize as needed. The latest stable version is 1.4.26, while the latest old stable version is 1.2.25.
https://xapian.org/