[DOCS] Rewrite regexp query

docs/reference/index-modules.asciidoc

@@ -205,6 +205,7 @@ specific index module:
     The maximum number of terms that can be used in Terms Query.
     Defaults to `65536`.
 
+[[index-max-regex-length]]
  `index.max_regex_length`::
 
     The maximum length of regex that can be used in Regexp Query.

docs/reference/query-dsl.asciidoc

@@ -47,4 +47,6 @@ include::query-dsl/term-level-queries.asciidoc[]
 
 include::query-dsl/minimum-should-match.asciidoc[]
 
-include::query-dsl/multi-term-rewrite.asciidoc[]
+include::query-dsl/multi-term-rewrite.asciidoc[]
+
+include::query-dsl/regexp-syntax.asciidoc[]

docs/reference/query-dsl/regexp-query.asciidoc

@@ -4,98 +4,86 @@
 <titleabbrev>Regexp</titleabbrev>
 ++++
 
-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.
+Returns documents that contain terms matching a
+https://en.wikipedia.org/wiki/Regular_expression[regular expression].
 
-*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.
+A regular expression is a way to match patterns in data using placeholder
+characters, called operators. For a list of operators supported by the
+`regexp` query, see <<regexp-syntax, Regular expression syntax>>.
 
-[source,js]
---------------------------------------------------
-GET /_search
-{
-    "query": {
-        "regexp":{
-            "name.first": "s.*y"
-        }
-    }
-}
---------------------------------------------------
-// CONSOLE
+[[regexp-query-ex-request]]
+==== Example request
 
-Boosting is also supported
+The following search returns documents where the `user` field contains any term
+that begins with `k` and ends with `y`. The `.*` operators match any
+characters of any length, including no characters. Matching
+terms can include `ky`, `kay`, and `kimchy`.
 
 [source,js]
---------------------------------------------------
+----
 GET /_search
 {
     "query": {
-        "regexp":{
-            "name.first":{
-                "value":"s.*y",
-                "boost":1.2
+        "regexp": {
+            "user": {
+                "value": "k.*y",
+                "flags" : "ALL",
+                "max_determinized_states": 10000,
+                "rewrite": "constant_score"
             }
         }
     }
 }
---------------------------------------------------
+----
 // CONSOLE
 
-You can also use special flags
 
-[source,js]
---------------------------------------------------
-GET /_search
-{
-    "query": {
-        "regexp":{
-            "name.first": {
-                "value": "s.*y",
-                "flags" : "INTERSECTION|COMPLEMENT|EMPTY"
-            }
-        }
-    }
-}
---------------------------------------------------
-// CONSOLE
+[[regexp-top-level-params]]
+==== Top-level parameters for `regexp`
+`<field>`::
+(Required, object) Field you wish to search.
 
-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
+[[regexp-query-field-params]]
+==== Parameters for `<field>`
+`value`::
+(Required, string) Regular expression for terms you wish to find in the provided
+`<field>`. For a list of supported operators, see <<regexp-syntax, Regular
+expression syntax>>.
++
+--
+By default, regular expressions are limited to 1,000 characters. You can change
+this limit using the <<index-max-regex-length, `index.max_regex_length`>>
+setting.
 
-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.
+[WARNING]
+=====
+The performance of the `regexp` query can vary based on the regular expression
+provided. To improve performance, avoid using wildcard patterns, such as `.*` or
+`.*?+`, without a prefix or suffix.
+=====
+--
 
-[source,js]
---------------------------------------------------
-GET /_search
-{
-    "query": {
-        "regexp":{
-            "name.first": {
-                "value": "s.*y",
-                "flags" : "INTERSECTION|COMPLEMENT|EMPTY",
-                "max_determinized_states": 20000
-            }
-        }
-    }
-}
---------------------------------------------------
-// CONSOLE
+`flags`::
+(Optional, string) Enables optional operators for the regular expression. For
+valid values and more information, see <<regexp-optional-operators, Regular
+expression syntax>>.
+
+`max_determinized_states`::
++
+--
+(Optional, integer) Maximum number of
+https://en.wikipedia.org/wiki/Deterministic_finite_automaton[automaton states]
+required for the query. Default is `10000`.
+
+{es} uses https://lucene.apache.org/core/[Apache Lucene] internally to parse
+regular expressions. Lucene converts each regular expression to a finite
+automaton containing a number of determinized states.
 
-NOTE: By default the maximum length of regex string allowed in a Regexp Query 
-is limited to 1000. You can update the `index.max_regex_length` index setting 
-to bypass this limit.
+You can use this parameter to prevent that conversion from unintentionally
+consuming too many resources. You may need to increase this limit to run complex
+regular expressions.
+--
 
-include::regexp-syntax.asciidoc[]
+`rewrite`::
+(Optional, string) Method used to rewrite the query. For valid values and more
+information, see the <<query-dsl-multi-term-rewrite, `rewrite` parameter>>.