[[query-dsl-regexp-query]]
=== Regexp Query

The `regexp` query allows you to use regular expression term queries.
See <<regexp-syntax>> for details of the supported regular expression language.
The "term queries" in that first sentence means that Elasticsearch will apply
the regexp to the terms produced by the tokenizer for that field, and not
to the original text of the field.

*Note*: The performance of a `regexp` query heavily depends on the
regular expression chosen. Matching everything like `.*` is very slow as
well as using lookaround regular expressions. If possible, you should
try to use a long prefix before your regular expression starts. Wildcard
matchers like `.*?+` will mostly lower performance.

[source,js]
--------------------------------------------------
{
    "regexp":{
        "name.first": "s.*y"
    }
}
--------------------------------------------------

Boosting is also supported

[source,js]
--------------------------------------------------
{
    "regexp":{
        "name.first":{
            "value":"s.*y",
            "boost":1.2
        }
    }
}
--------------------------------------------------

You can also use special flags

[source,js]
--------------------------------------------------
{
    "regexp":{
        "name.first": {
            "value": "s.*y",
            "flags" : "INTERSECTION|COMPLEMENT|EMPTY"
        }
    }
}
--------------------------------------------------

Possible flags are `ALL` (default), `ANYSTRING`, `COMPLEMENT`,
`EMPTY`, `INTERSECTION`, `INTERVAL`, or `NONE`. Please check the
http://lucene.apache.org/core/4_9_0/core/org/apache/lucene/util/automaton/RegExp.html[Lucene
documentation] for their meaning

Regular expressions are dangerous because it's easy to accidentally
create an innocuous looking one that requires an exponential number of
internal determinized automaton states (and corresponding RAM and CPU)
for Lucene to execute.  Lucene prevents these using the
`max_determinized_states` setting (defaults to 10000).  You can raise
this limit to allow more complex regular expressions to execute.

[source,js]
--------------------------------------------------
{
    "regexp":{
        "name.first": {
            "value": "s.*y",
            "flags" : "INTERSECTION|COMPLEMENT|EMPTY",
	    "max_determinized_states": 20000
        }
    }
}
--------------------------------------------------


include::regexp-syntax.asciidoc[]
