In this post, I'll show you what you need to do to implement a custom Solr QueryParser.

Step 1

Extend QParserPlugin.
public class TestQueryParserPlugin extends QParserPlugin {
  public void init(NamedList namedList) {
  }
  @Override public QParser createParser(String s, SolrParams localParams, SolrParams params, SolrQueryRequest req) {
    return new TestQParser(s, localParams, params, req);
  }
}
 
This is the class you'll define in solrconfig.xml, informing Solr of your queryparser. Define it like so:
<queryParser name="myfunparser"class="org.supermind.solr.queryparser.TestQParserPlugin"/>
 

Step 2

Extend QParser.
  public class TestQParser extends QParser {
    public TestQParser(String qstr, SolrParams localParams, SolrParams params, SolrQueryRequest req) {
      super(qstr, localParams, params, req);
    }
    @Override public Query parse() throws SyntaxError {
      return null;
    }
  }
 

Step 3

Actually implement the parsing in the parse() method.
Suppose we wanted to make a really simple parser for term queries, which are space-delimited. Here's how I'd do it:
@Override public Query parse() throws SyntaxError {
      String defaultField = req.getSchema().getDefaultSearchFieldName();
      QueryParser.Operator defaultOperator = QueryParser.Operator.valueOf(req.getSchema().getQueryParserDefaultOperator());
      BooleanClause.Occur op = (defaultOperator == QueryParser.Operator.AND) ? BooleanClause.Occur.MUST : BooleanClause.Occur.SHOULD;
      String[] arr = qstr.split(" ");
      BooleanQuery bq = new BooleanQuery(true);
      for(String s: arr) {
        if(s.trim().length() == 0) continue;
        bq.add(new TermQuery(new Term(defaultField, s)), op);
      }
      return bq;
    }
 

Step 4

In your query, use the nested query syntax to call your queryparser, e.g.
http://localhost:8983/solr/collection1/select?q={!myfunparser}foo+bar+car
Maybe in a follow-up post, I'll post the full code with jars and all.