# Xapian::QueryParser Syntax

This document describes the query syntax supported by the Xapian::QueryParser class. The syntax is designed to be similar to other web based search engines, so that users familiar with them don't have to learn a whole new syntax.

# Operators

## AND

expression AND expression matches documents which are matched by both of the subexpressions.

## OR

expression OR expression matches documents which are matched by either of the subexpressions.

## NOT

expression NOT expression matches documents which are matched by only the first subexpression. This can also be written as expression AND NOT expression. If `FLAG_PURE_NOT` is enabled, then NOT expression will match documents which don't match the subexpression.

## XOR

expression XOR expression matches documents which are matched by one or other of the subexpressions, but not both. XOR is probably a bit esoteric.

## Bracketed expressions

You can control the precedence of the boolean operators using brackets. In the query `one OR two AND three` the AND takes precedence, so this is the same as `one OR (two AND three)`. You can override the precedence using `(one OR two) AND three`.

The default precedence from highest to lowest is:

- +, - (equal)
- NEAR, ADJ (equal)
- AND, NOT (equal)
- XOR
- OR

## '+' and '-'

A group of terms with some marked with + and - will match documents containing all of the + terms, but none of the - terms. Terms not marked with + or - contribute towards the document rankings. You can also use + and - on phrases and on bracketed expressions.

## NEAR

`one NEAR two NEAR three` matches documents containing those words within 10 words of each other. You can set the threshold to n by using `NEAR/n` like so: `one NEAR/6 two`.

## ADJ

`ADJ` is like `NEAR` but only matches if the words appear in the same order as in the query. So `one ADJ two ADJ three` matches documents containing those three words in that order and within 10 words of each other. You can set the threshold to n by using `ADJ/n` like so: `one ADJ/6 two`.

## Phrase searches

A phrase surrounded with double quotes ("") matches documents containing that exact phrase. Hyphenated words are also treated as phrases, as are cases such as filenames and email addresses (e.g. `/etc/passwd` or `president@whitehouse.gov`).

## Searching within a free-text field

If the database has been indexed with prefixes on terms generated from certain free-text fields, you can set up a prefix map so that the user can search within those fields. For example `author:dickens title:shop` might find documents by dickens with shop in the title. You can also specify a prefix on a quoted phrase (e.g. `author:"charles dickens"`) or on a bracketed subexpression (e.g. `title:(mice men)`).

## Searching for proper names

If a query term is entered with a capitalised first letter, then it will be searched for unstemmed.

## Range searches

The QueryParser can be configured to support range-searching using document values.

The syntax for a range search is `start..end` - for example, `01/03/2007..04/04/2007`, `$10..100`, `5..10kg`.

Open-ended ranges are also supported - an empty start or end is interpreted as no limit, for example: `..2010-06-17`, `$10..`, `$..100`, `..5kg`.

## Synonyms

The QueryParser can be configured to support synonyms, which can either be used when explicitly specified (using the syntax `~term`) or implicitly (synonyms will be used for all terms or groups of terms for which they have been specified).

## Wildcards

The QueryParser supports using a trailing '*' wildcard, which matches any number of trailing characters, so `wildc*` would match wildcard, wildcarded, wildcards, wildcat, wildcats, etc. This feature is disabled by default - pass `Xapian::QueryParser::FLAG_WILDCARD` in the flags argument of `Xapian::QueryParser::parse_query(query_string, flags)` to enable it, and tell the QueryParser which database to expand wildcards from using the `QueryParser::set_database(database)` method.

You can limit the number of terms a wildcard will expand to by calling `Xapian::QueryParser::set_max_expansion()`. This supports several different modes, and can also be used to limit expansion performed via `FLAG_PARTIAL` - see the API documentation for details. By default, there's no limit on wildcard expansion and `FLAG_PARTIAL` expands to the most frequent 100 terms.

## Partially entered query matching

The QueryParser also supports performing a search with a query which has only been partially entered. This is intended for use with "incremental search" systems, which don't wait for the user to finish typing their search before displaying an initial set of results. For example, in such a system a user would enter a search, and the system would display a new set of results after each letter, or whenever the user pauses for a short period of time (or some other similar strategy).

The problem with this kind of search is that the last word in a partially entered query often has no semantic relation to the completed word. For example, a search for "dynamic cat" would return a quite different set of results to a search for "dynamic categorisation". This results in the set of results displayed flicking rapidly as each new character is entered. A much smoother result can be obtained if the final word is treated as having an implicit terminating wildcard, so that it matches all words starting with the entered characters - thus, as each letter is entered, the set of results displayed narrows down to the desired subject.

A similar effect could be obtained simply by enabling the wildcard matching option, and appending a "*" character to each query string. However, this would be confused by searches which ended with punctuation or other characters.

This feature is disabled by default - pass `Xapian::QueryParser::FLAG_PARTIAL` flag in the flags argument of `Xapian::QueryParser::parse_query(query_string, flags)` to enable it, and tell the QueryParser which database to expand wildcards from using the `QueryParser::set_database(database)` method.