## Apache Lucene Flexible Query Parser

This contrib project contains the new Lucene query parser implementation, which matches the syntax of the core QueryParser but offers a more modular architecture to enable customization.

It's currently divided in 2 main packages:

- {@link org.apache.lucene.queryParser.core}: it contains the query parser API classes, which should be extended by query parser implementations.
- {@link org.apache.lucene.queryParser.standard}: it contains the current Lucene query parser implementation using the new query parser API.

### Features

1. Full support for boolean logic (not enabled)
2. QueryNode Trees - support for several syntaxes, that can be converted into similar syntax QueryNode trees.
3. QueryNode Processors - Optimize, validate, rewrite the QueryNode trees
4. Processors Pipelines - Select your favorite Processor and build a processor pipeline, to implement the features you need
5. Config Interfaces - Allow the consumer of the Query Parser to implement a diff Config Handler Objects to suite their needs.
6. Standard Builders - convert QueryNode's into several lucene representations. Supported conversion is using a 2.4 compatible logic
7. QueryNode tree's can be converted to a lucene 2.4 syntax string, using toQueryString

### Design

This new query parser was designed to have very generic architecture, so that it can be easily used for different products with varying query syntaxes. This code is much more flexible and extensible than the Lucene query parser in 2.4.X.

The new query parser goal is to separate syntax and semantics of a query. E.g. 'a AND b', '+a +b', 'AND(a,b)' could be different syntaxes for the same query. It distinguishes the semantics of the different query components, e.g. whether and how to tokenize/lemmatize/normalize the different terms or which Query objects to create for the terms. It allows to write a parser with a new syntax, while reusing the underlying semantics, as quickly as possible.

The query parser has three layers and its core is what we call the QueryNode tree. It is a tree that initially represents the syntax of the original query, e.g. for 'a AND b':

          AND
         /   \
        A     B

The three layers are:

QueryParser
  This layer is the text parsing layer which simply transforms the query text string into a {@link org.apache.lucene.queryParser.core.nodes.QueryNode} tree. Every text parser must implement the interface {@link org.apache.lucene.queryParser.core.parser.SyntaxParser}. Lucene default implementations implements it using JavaCC.
QueryNodeProcessor
  The query node processors do most of the work. It is in fact a configurable chain of processors. Each processors can walk the tree and modify nodes or even the tree's structure. That makes it possible to e.g. do query optimization before the query is executed or to tokenize terms.
QueryBuilder
  The third layer is a configurable map of builders, which map {@link org.apache.lucene.queryParser.core.nodes.QueryNode} types to its specific builder that will transform the QueryNode into Lucene Query object.

Furthermore, the query parser uses flexible configuration objects. It also uses message classes that allow to attach resource bundles. This makes it possible to translate messages, which is an important feature of a query parser.

This design allows to develop different query syntaxes very quickly.

### StandardQueryParser and QueryParserWrapper

The standard (default) Lucene query parser is located under org.apache.lucene.queryParser.standard.

To make it simpler to use the new query parser the class {@link org.apache.lucene.queryParser.standard.StandardQueryParser} may be helpful, specially for people that do not want to extend the Query Parser. It uses the default Lucene query processors, text parser and builders, so you don't need to worry about dealing with those. {@link org.apache.lucene.queryParser.standard.StandardQueryParser} usage:

          StandardQueryParser qpHelper = new StandardQueryParser();
          StandardQueryConfigHandler config =  qpHelper.getQueryConfigHandler();
          config.setAllowLeadingWildcard(true);
          config.setAnalyzer(new WhitespaceAnalyzer());
          Query query = qpHelper.parse("apache AND lucene", "defaultField");

To make it easy for people who are using current Lucene's query parser to switch to the new one, there is a {@link org.apache.lucene.queryParser.standard.QueryParserWrapper} under org.apache.lucene.queryParser.standard that keeps the old query parser interface, but uses the new query parser infrastructure.