Xapian

Features

• Supports Unicode 9.0 and stores indexed text in UTF-8.
• Transactions: if database update fails in the middle of a transaction, the database is guaranteed to remain in a consistent state.
• Simultaneous search and update, with new documents being immediately visible.
• Support for large databases: Xapian has been proven to scale to hundreds of millions of documents.
• Accurate probabilistic ranking: more relevant documents are listed first.
• Phrase and proximity searching.
• Relevance feedback, which improves ranking and can expand a query, find related documents, categorise documents etc.
• Structured Boolean queries, e.g. "race AND condition NOT horse"
• Wildcard search, e.g. "wiki*"
• Spelling correction
• Synonyms
• Omega, a packaged solution for adding a search engine to a web site or intranet. Omega can easily be extended and adapted to fit changing requirements.