[2012-01-11: Updated version available.]

There's often a need to modify queries in different ways, like changing every word in a search string into a prefix search (word => word*). A common thought is to do this using regular expressions, or basic string mangling. This, however, is hard to get right when you take into consideration the many different query types, and their syntax. A easier way to solve this is to rewrite the query generated by the QueryParser. This needs to be done recursivly since queries can nest other queries ("(a b)" is a boolean query with two nested term queries). I've attached a QueryVisitor class which simplifies this.

An example; to change every TermQuery (word) into a PrefixQuery (word*), we just need to override the VisitTermQuery method, and return a PrefixQuery instead. This wont match phrase queries ("a word"), but those often mean that the user wants the exact spelling/formulation specified.

public class PrefixRewriter : QueryVisitor {
    protected override Query VisitTermQuery(TermQuery query) {
        var term = query.GetTerm();
        var newQuery = new PrefixQuery(term);
        return CopyBoost(query, newQuery);
    }
}

Note that phrases with only one term ("word") are parsed as a TermQuery and thus rewritten. This is done after the analyzer has removed all stopwords, causing phrases like "with a word" to become a search for a single term, "word", which is rewritten in our example. 

public static class Program {
    public static void Main() {
        var queryParser = new QueryParser("f", new StandardAnalyzer());
        var query = queryParser.Parse("awesome rewrite^0.5 \"including one phrase\"");
        var rewritten = new PrefixRewriter().Visit(query);

        Console.WriteLine(query);
        Console.WriteLine(rewritten);
    }
}

Outputs...

f:awesome f:rewrite^0.5 f:"including one phrase"
f:awesome* f:rewrite*^0.5 f:"including one phrase"

Download: QueryVisitor.cs (5.52 kb)