Get matched terms from Lucene query

Question

Given a Lucene search query like: +(letter:A letter:B letter:C) +(style:Capital), how can I tell which of the three letters actually matched any given document?

Answer 1

Here is a simplified and non-recursive version with Lucene.NET 4.8.
Unverified, but this should also work on Lucene.NET 3.x

IEnumerable GetHitTermsForDoc(Query query, IndexSearcher searcher, int docId)
{
    //Rewrite query into simpler internal form, required for ExtractTerms
    var simplifiedQuery = query.Rewrite(searcher.IndexReader);
    HashSet queryTerms = new HashSet();
    simplifiedQuery.ExtractTerms(queryTerms);

    List hitTerms = new List();
    foreach (var term in queryTerms)
    {
        var termQuery = new TermQuery(term);

        var explanation = searcher.Explain(termQuery, docId);
        if (explanation.IsMatch)
        {
            hitTerms.Add(term);
        }
    }
    return hitTerms;
}