src/Lucene.Net.QueryParser/Flexible/Core/package.md

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uid: Lucene.Net.QueryParsers.Flexible.Core
summary: *content
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Core classes of the flexible query parser framework.

## Flexible Query Parser

This namespace contains the necessary classes to implement a query parser.

A query parser is divided in at least 2 phases, text parsing and query building, and one optional phase called query processing.

### First Phase: Text Parsing

The text parsing phase is performed by a text parser, which implements <xref:Lucene.Net.QueryParsers.Flexible.Core.Parser.ISyntaxParser> interface. A text parser is responsible to get a query string and convert it to a <xref:Lucene.Net.QueryParsers.Flexible.Core.Nodes.IQueryNode> tree, which is an object structure that represents the elements defined in the query string.

### Second (optional) Phase: Query Processing

The query processing phase is performed by a query processor, which implements <xref:Lucene.Net.QueryParsers.Flexible.Core.Processors.QueryNodeProcessor>. A query processor is responsible to perform any processing on a <xref:Lucene.Net.QueryParsers.Flexible.Core.Nodes.IQueryNode> tree. This phase is optional and is used only if an extra processing, validation, query expansion, etc needs to be performed in a <xref:Lucene.Net.QueryParsers.Flexible.Core.Nodes.IQueryNode> tree. The <xref:Lucene.Net.QueryParsers.Flexible.Core.Nodes.IQueryNode> tree can be either be generated by a text parser or programmatically created.

### Third Phase: Query Building

The query building phase is performed by a query builder, which implements <xref:Lucene.Net.QueryParsers.Flexible.Core.Builders.IQueryBuilder{Lucene.Net.QueryParsers.Flexible.Core.Nodes.IQueryNode}>. A query builder is responsible to convert a <xref:Lucene.Net.QueryParsers.Flexible.Core.Nodes.IQueryNode> tree into an arbitrary object, which is usually used to be executed against a search index.