[DOC] Add gsub processor documentation

_ingest-pipelines/processors/gsub.md

@@ -0,0 +1,170 @@
+---
+layout: default
+title: gsub
+parent: Ingest processors
+nav_order: 130
+---
+
+# Gsub processor
+
+The `gsub` processor performs a regular expression search-and-replace operation on string fields in incoming documents. If the field contains an array of strings, the operation is applied to all elements in the array. However, if the field contains non-string values, the processor throws an exception. Use cases for the `gsub` processor include removing sensitive information from log messages or user-generated content, normalizing data formats or conventions (for example, converting date formats, removing special characters), and extracting or transforming substrings from field values for further processing or analysis.
+
+The following is the syntax for the `gsub` processor:
+
+```json
+"gsub": {
+  "field": "field_name",
+  "pattern": "regex_pattern",
+  "replacement": "replacement_string"
+}
+```
+{% include copy-curl.html %}
+
+## Configuration parameters
+
+The following table lists the required and optional parameters for the `gsub` processor.
+
+Parameter | Required/Optional | Description |
+|-----------|-----------|-----------|
+`field` | Required | The field to apply the replacement to.
+`pattern` | Required | The pattern to be replaced.
+`replacement` | Required | The string that will replace the matching patterns.
+`target_field` | Optional | The name of the field in which to store the parsed data. If `target_field` is not specified, the parsed data replaces the original data in the `field` field. Default is `field`.
+`if` | Optional | A condition for running the processor.
+`ignore_missing` | Optional | Specifies whether the processor should ignore documents that do not contain the specified field. Default is `false`.
+`ignore_failure` | Optional | Specifies whether the processor continues execution even if it encounters an error. If set to `true`, then failures are ignored. Default is `false`.
+`on_failure` | Optional | A list of processors to run if the processor fails.
+`tag` | Optional | An identifier tag for the processor. Useful for debugging in order to distinguish between processors of the same type.
+
+## Using the processor
+
+Follow these steps to use the processor in a pipeline.
+
+### Step 1: Create a pipeline
+
+The following query creates a pipeline named `gsub_pipeline` that uses the `gsub` processor to replace all occurrences of the word `error` with the word `warning` in the `message` field:
+
+```json
+PUT _ingest/pipeline/gsub_pipeline
+{
+  "description": "Replaces 'error' with 'warning' in the 'message' field",
+  "processors": [
+    {
+      "gsub": {
+        "field": "message",
+        "pattern": "error",
+        "replacement": "warning"
+      }
+    }
+  ]
+}
+```
+{% include copy-curl.html %}
+
+### Step 2 (Optional): Test the pipeline
+
+It is recommended that you test your pipeline before you ingest documents.
+{: .tip}
+
+To test the pipeline, run the following query:
+
+```json
+POST _ingest/pipeline/gsub_pipeline/_simulate
+{
+  "docs": [
+    {
+      "_source": {
+        "message": "This is an error message"
+      }
+    }
+  ]
+}
+```
+{% include copy-curl.html %}
+
+#### Response
+
+The following response confirms that the pipeline is working as expected:
+
+```json
+{
+  "docs": [
+    {
+      "doc": {
+        "_index": "_index",
+        "_id": "_id",
+        "_source": {
+          "message": "This is an warning message"
+        },
+        "_ingest": {
+          "timestamp": "2024-05-22T19:47:00.645687211Z"
+        }
+      }
+    }
+  ]
+}
+```
+{% include copy-curl.html %}
+
+### Step 3: Ingest a document 
+
+The following query ingests a document into an index named `logs`:
+
+```json
+PUT logs/_doc/1?pipeline=gsub_pipeline
+{
+  "message": "This is an error message"
+}
+```
+{% include copy-curl.html %}
+
+#### Response
+
+The following response shows that the request indexed the document into the index named `logs` and that the `gsub` processor replaced all occurrences of the word `error` with the word `warning` in the `message` field:
+
+```json
+{
+  "_index": "logs",
+  "_id": "1",
+  "_version": 1,
+  "result": "created",
+  "_shards": {
+    "total": 2,
+    "successful": 1,
+    "failed": 0
+  },
+  "_seq_no": 0,
+  "_primary_term": 1
+}
+```
+{% include copy-curl.html %}
+
+### Step 4 (Optional): Retrieve the document
+
+To retrieve the document, run the following query:
+
+```json
+GET logs/_doc/1
+```
+{% include copy-curl.html %}
+
+#### Response
+
+The following response shows the document with the modified `message` field value:
+
+```json
+{
+  "_index": "logs",
+  "_id": "1",
+  "_version": 1,
+  "_seq_no": 0,
+  "_primary_term": 1,
+  "found": true,
+  "_source": {
+    "message": "This is an warning message"
+  }
+}
+```
+{% include copy-curl.html %}
+
+