Apache Lucene

Apache Lucene is an unprecedentedly efficient open-source information retrieval Java library. With the concept of a text field found within any document, the technology is widely known for the ability to remain effective at indexing and extracting textual data from files with no regard to their format (Word, PDF, HTML, etc.), thus providing the full-text search flexibility especially valuable for the development of both local site search systems and standalone search engines. Lucene is known to be currently used by Twitter, Inc.