Lucene

Lucene is the high-performance, open-source Java library that provides the core indexing and search engine capabilities for applications like Elasticsearch and Apache Solr.

Lucene Core is the definitive, high-performance, full-featured search engine library: it is 100% Java and operates under the Apache License 2.0 (ASL). It is the foundational technology—the 'search brain'—for major platforms including Elasticsearch and Apache Solr. Lucene handles both indexing and querying, using an efficient inverted index structure to manage data. Key features include a powerful API for complex query types (e.g., phrase, wildcard, fuzzy) and high-speed indexing (over 800GB/hour on modern hardware), making it the de facto standard for building custom, scalable full-text search into any application.

https://lucene.apache.org/