[Backport 2.18] add pattern analyzer docs

_analyzers/pattern.md

@@ -0,0 +1,96 @@
+---
+layout: default
+title: Pattern analyzer
+nav_order: 90
+---
+
+# Pattern analyzer
+
+The `pattern` analyzer allows you to define a custom analyzer that uses a regular expression (regex) to split input text into tokens. It also provides options for applying regex flags, converting tokens to lowercase, and filtering out stopwords.
+
+## Parameters
+
+The `pattern` analyzer can be configured with the following parameters.
+
+Parameter | Required/Optional | Data type | Description
+:--- | :--- | :--- | :--- 
+`pattern` | Optional | String | A [Java regular expression](https://docs.oracle.com/javase/8/docs/api/java/util/regex/Pattern.html) used to tokenize the input. Default is `\W+`.
+`flags` | Optional | String | A string containing pipe-separated [Java regex flags](https://docs.oracle.com/javase/8/docs/api/java/util/regex/Pattern.html#field.summary) that modify the behavior of the regular expression.
+`lowercase` | Optional | Boolean | Whether to convert tokens to lowercase. Default is `true`.
+`stopwords` | Optional | String or list of strings | A string specifying a predefined list of stopwords (such as `_english_`) or an array specifying a custom list of stopwords. Default is `_none_`.
+`stopwords_path` | Optional | String | The path (absolute or relative to the config directory) to the file containing a list of stopwords.
+
+
+## Example
+
+Use the following command to create an index named `my_pattern_index` with a `pattern` analyzer:
+
+```json
+PUT /my_pattern_index
+{
+  "settings": {
+    "analysis": {
+      "analyzer": {
+        "my_pattern_analyzer": {
+          "type": "pattern",
+          "pattern": "\\W+",  
+          "lowercase": true,                
+          "stopwords": ["and", "is"]       
+        }
+      }
+    }
+  },
+  "mappings": {
+    "properties": {
+      "my_field": {
+        "type": "text",
+        "analyzer": "my_pattern_analyzer"
+      }
+    }
+  }
+}
+```
+{% include copy-curl.html %}
+
+## Generated tokens
+
+Use the following request to examine the tokens generated using the analyzer:
+
+```json
+POST /my_pattern_index/_analyze
+{
+  "analyzer": "my_pattern_analyzer",
+  "text": "OpenSearch is fast and scalable"
+}
+```
+{% include copy-curl.html %}
+
+The response contains the generated tokens:
+
+```json
+{
+  "tokens": [
+    {
+      "token": "opensearch",
+      "start_offset": 0,
+      "end_offset": 10,
+      "type": "word",
+      "position": 0
+    },
+    {
+      "token": "fast",
+      "start_offset": 14,
+      "end_offset": 18,
+      "type": "word",
+      "position": 2
+    },
+    {
+      "token": "scalable",
+      "start_offset": 23,
+      "end_offset": 31,
+      "type": "word",
+      "position": 4
+    }
+  ]
+}
+```