Adding example to contains data prepper

_data-prepper/pipelines/contains.md

@@ -18,7 +18,7 @@
 For example, if you want to check if the string `"abcd"` is contained within the value of a field named `message`, you can use the `contains()` function as follows:
 
 ```
-contains('/message', 'abcd')
+'contains(/message, "abcd")'
 ```
 {% include copy.html %}
 
@@ -27,11 +27,89 @@
 Alternatively, you can use a literal string as the first argument:
 
 ```
-contains('This is a test message', 'test')
+'contains("This is a test message", "test")'
 ```
 {% include copy.html %}
 
 In this case, the function returns `true` because the substring `test` is present within the string `This is a test message`.
 
 The `contains()` function performs a case-sensitive search.
 {: .note}
+
+## Example
+
+The following pipeline uses `contains()` to add a boolean flag `has_test` based on a substring in `/message` and to filter out non-matching events, forwarding only messages containing "ERROR" to OpenSearch:
+
+```yaml
+contains-demo-pipeline:
+  source:
+    http:
+      ssl: false
+
+  processor:
+    - add_entries:
+        entries:
+          - key: "has_test"
+            value_expression: 'contains(/message, "test")'
+    - drop_events:
+        drop_when: 'not contains(/message, "ERROR")'
+
+  sink:
+    - opensearch:
+        hosts: ["https://opensearch:9200"]
+        insecure: true
+        username: admin
+        password: "admin_pass"
+        index_type: custom
+        index: "demo-index-%{yyyy.MM.dd}"
+```
+{% include copy.html %}
+
+You can test the pipeline using the following command:
+
+```bash
+curl -sS -X POST "http://localhost:2021/events" \
+  -H "Content-Type: application/json" \
+  -d '[
+        {"message":"ok hello"},                  
+        {"message":"this has test but ok"},
+        {"message":"ERROR: something bad"}, 
+        {"message":"ERROR: unit test failed"} 
+      ]'
+```
+{% include copy.html %}
+
+The documents stored in OpenSearch contain the following information:
+
+```json
+{
+  ...
+  "hits": {
+    "total": {
+      "value": 2,
+      "relation": "eq"
+    },
+    "max_score": 1,
+    "hits": [
+      {
+        "_index": "demo-index-2025.10.21",
+        "_id": "5YACB5oBqZitdAAb4n3r",
+        "_score": 1,
+        "_source": {
+          "message": "ERROR: something bad",
+          "has_test": false
+        }
+      },
+      {
+        "_index": "demo-index-2025.10.21",
+        "_id": "5oACB5oBqZitdAAb4n3r",
+        "_score": 1,
+        "_source": {
+          "message": "ERROR: unit test failed",
+          "has_test": true
+        }
+      }
+    ]
+  }
+}
+```