Fix backslash escaping in Hive string literals for Trino conversion

coral-hive/src/main/java/com/linkedin/coral/hive/hive2rel/parsetree/ParseTreeBuilder.java

@@ -692,28 +692,29 @@ protected SqlNode visitIdentifier(ASTNode node, ParseContext ctx) {
     return new SqlIdentifier(node.getText(), ZERO);
   }
 
-  /** See {@link #removeBackslashBeforeQuotes}
-   * We use removeBackslashBeforeQuotes to remove the backslash before quotes,
-   * so that we maintain patterns like {@code I'm} or {@code abc"xyz} as is in the java object in memory,
-   * the escaped literal string representation will be generated when the SqlNode is written to string
+  /** See {@link #unescapeHiveStringLiteral}
+   * We use unescapeHiveStringLiteral to interpret Hive backslash escape sequences,
+   * so that patterns like {@code \\d} are correctly stored as {@code \d} in the java object in memory.
+   * The escaped literal string representation will be generated when the SqlNode is written to string
    * by the SqlWriter, which can be controlled by the SqlDialect to decide the choice of escaping mechanism.
    * */
   @Override
   protected SqlNode visitStringLiteral(ASTNode node, ParseContext ctx) {
     // TODO: Add charset here. UTF-8 is not supported by calcite
     String text = node.getText();
     checkState(text.length() >= 2);
-    return SqlLiteral.createCharString(removeBackslashBeforeQuotes(text.substring(1, text.length() - 1)), ZERO);
+    return SqlLiteral.createCharString(unescapeHiveStringLiteral(text.substring(1, text.length() - 1)), ZERO);
   }
 
-  private String removeBackslashBeforeQuotes(String input) {
-    // matches a \' or \" literal pattern
-    Pattern pattern = Pattern.compile("\\\\['\"]");
+  private String unescapeHiveStringLiteral(String input) {
+    // Handle Hive backslash escape sequences: \\ -> \, \' -> ', \" -> "
+    Pattern pattern = Pattern.compile("\\\\[\\\\'\"]");
     Matcher matcher = pattern.matcher(input);
 
     StringBuffer res = new StringBuffer();
     while (matcher.find()) {
-      String replacement = matcher.group().substring(1);
+      String matched = matcher.group();
+      String replacement = Matcher.quoteReplacement(matched.substring(1));
       matcher.appendReplacement(res, replacement);
     }
     matcher.appendTail(res);

coral-trino/src/test/java/com/linkedin/coral/trino/rel2trino/HiveToTrinoConverterTest.java

@@ -27,6 +27,8 @@
 import static com.linkedin.coral.trino.rel2trino.CoralTrinoConfigKeys.*;
 import static org.apache.calcite.sql.type.OperandTypes.*;
 import static org.testng.Assert.assertEquals;
+import static org.testng.Assert.assertFalse;
+import static org.testng.Assert.assertTrue;
 
 
 public class HiveToTrinoConverterTest {
@@ -966,6 +968,55 @@ public void testRegexpTransformation() {
     assertEquals(expandedSql, targetSql);
   }
 
+  @Test
+  public void testRlikeBackslashEscaping() {
+    RelToTrinoConverter relToTrinoConverter = TestUtils.getRelToTrinoConverter();
+
+    // In Hive SQL, '\\d' means the string \d (backslash is escape char).
+    // In Trino SQL, '\d' means the string \d (no backslash escaping).
+    // So Hive's '\\d{4}' should become Trino's '\d{4}'.
+    RelNode relNode =
+        TestUtils.getHiveToRelConverter().convertSql("SELECT '2022-01-01' RLIKE '^\\\\d{4}-\\\\d{2}-\\\\d{2}$'");
+    String targetSql =
+        "SELECT \"REGEXP_LIKE\"('2022-01-01', '^\\d{4}-\\d{2}-\\d{2}$')\n" + "FROM (VALUES  (0)) AS \"t\" (\"ZERO\")";
+    String expandedSql = relToTrinoConverter.convert(relNode);
+    assertEquals(expandedSql, targetSql);
+  }
+
+  @Test
+  public void testRlikeBackslashEscapingWithColumn() {
+    RelToTrinoConverter relToTrinoConverter = TestUtils.getRelToTrinoConverter();
+
+    // Test backslash escaping with a column reference instead of literal
+    RelNode relNode =
+        TestUtils.getHiveToRelConverter().convertSql("SELECT * FROM test.tableA WHERE a RLIKE '^\\\\d+$'");
+    String expandedSql = relToTrinoConverter.convert(relNode);
+    assertTrue(expandedSql.contains("\"REGEXP_LIKE\""));
+    assertTrue(expandedSql.contains("'^\\d+$'"));
+    assertFalse(expandedSql.contains("'^\\\\d+$'"));
+  }
+
+  @Test
+  public void testRegexpBackslashEscaping() {
+    RelToTrinoConverter relToTrinoConverter = TestUtils.getRelToTrinoConverter();
+
+    // Test that REGEXP (synonym for RLIKE) also handles backslash escaping
+    RelNode relNode = TestUtils.getHiveToRelConverter().convertSql("SELECT 'hello' REGEXP '^\\\\w+$'");
+    String targetSql = "SELECT \"REGEXP_LIKE\"('hello', '^\\w+$')\n" + "FROM (VALUES  (0)) AS \"t\" (\"ZERO\")";
+    String expandedSql = relToTrinoConverter.convert(relNode);
+    assertEquals(expandedSql, targetSql);
+  }
+
+  @Test
+  public void testStringLiteralWithEscapedBackslash() {
+    RelToTrinoConverter relToTrinoConverter = TestUtils.getRelToTrinoConverter();
+
+    // Test that a literal backslash (\\\\  in Hive = \\ in string = one backslash in Trino)
+    RelNode relNode = TestUtils.getHiveToRelConverter().convertSql("SELECT 'path\\\\to\\\\file'");
+    String expandedSql = relToTrinoConverter.convert(relNode);
+    assertTrue(expandedSql.contains("'path\\to\\file'"));
+  }
+
   @Test
   public void testSqlSelectAliasAppenderTransformer() {
     // test.tableA(a int, b struct<b1:string>