add IT for regex cmd

Signed-off-by: Jialiang Liang <jiallian@amazon.com>

Author: RyanL1997

integ-test/src/test/java/org/opensearch/sql/ppl/RegexCommandIT.java

@@ -0,0 +1,91 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package org.opensearch.sql.ppl;
+
+import static org.opensearch.sql.legacy.TestsConstants.*;
+
+import java.io.IOException;
+import org.json.JSONObject;
+import org.junit.jupiter.api.Test;
+
+public class RegexCommandIT extends PPLIntegTestCase {
+
+  @Override
+  public void init() throws Exception {
+    super.init();
+    enableCalcite();
+    loadIndex(Index.ACCOUNT);
+  }
+
+  @Test
+  public void testRegexBasicStringMatch() throws IOException {
+    JSONObject result =
+        executeQuery(
+            String.format(
+                "source=%s | regex firstname='Amber' | fields account_number, firstname",
+                TEST_INDEX_ACCOUNT));
+
+    assertEquals(1, result.getJSONArray("datarows").length());
+    assertEquals("Amber", result.getJSONArray("datarows").getJSONArray(0).get(1));
+  }
+
+  @Test
+  public void testRegexPartialStringMatch() throws IOException {
+    JSONObject result =
+        executeQuery(
+            String.format(
+                "source=%s | regex firstname='nan' | fields account_number, firstname",
+                TEST_INDEX_ACCOUNT));
+
+    // Should match names containing "nan": Fernandez, Buchanan
+    assertEquals(2, result.getJSONArray("datarows").length());
+    // Verify one of the results contains "nan"
+    String firstName = result.getJSONArray("datarows").getJSONArray(0).get(1).toString();
+    assertTrue(firstName.contains("nan"));
+  }
+
+  @Test
+  public void testRegexPatternMatch() throws IOException {
+    JSONObject result =
+        executeQuery(
+            String.format(
+                "source=%s | regex firstname='A.*' | fields account_number, firstname",
+                TEST_INDEX_ACCOUNT));
+
+    // Should match names starting with A - there are 66 such names in accounts.json
+    assertEquals(66, result.getJSONArray("datarows").length());
+    // Verify first result is a name starting with A
+    assertTrue(result.getJSONArray("datarows").getJSONArray(0).get(1).toString().startsWith("A"));
+  }
+
+  @Test
+  public void testRegexNegatedMatch() throws IOException {
+    JSONObject result =
+        executeQuery(
+            String.format(
+                "source=%s | regex firstname!='Amber' | fields account_number, firstname | head 3",
+                TEST_INDEX_ACCOUNT));
+
+    assertEquals(3, result.getJSONArray("datarows").length());
+    // Verify Amber is not in results
+    for (int i = 0; i < result.getJSONArray("datarows").length(); i++) {
+      assertNotEquals("Amber", result.getJSONArray("datarows").getJSONArray(i).get(1));
+    }
+  }
+
+  @Test
+  public void testRegexWithStateField() throws IOException {
+    JSONObject result =
+        executeQuery(
+            String.format(
+                "source=%s | regex state='CA' | fields account_number, firstname, state",
+                TEST_INDEX_ACCOUNT));
+
+    // There are 17 CA records in accounts.json
+    assertEquals(17, result.getJSONArray("datarows").length());
+    assertEquals("CA", result.getJSONArray("datarows").getJSONArray(0).get(2));
+  }
+}