test(datasync): add edge case tests for RegexTransformer

Tests cover:
- Capture group substitution in replacement
- Case-insensitive matching
- Null/DBNull passthrough
- Non-target columns unchanged
- Invalid regex pattern handling
- Column not found handling

NEW/tests/JdeScoping.DataSync.Tests/Etl/Transformers/RegexTransformerTests.cs

@@ -1,4 +1,5 @@
 using System.Data;
+using System.Text.RegularExpressions;
 using JdeScoping.DataSync.Etl.Transformers;
 using NSubstitute;
 
@@ -120,6 +121,132 @@ public class RegexTransformerTests
         Assert.Equal(string.Empty, reader.GetValue(0));
     }
 
+    [Fact]
+    public void FindReplace_UseCaptureGroups()
+    {
+        // Arrange - swap two numbers
+        var source = CreateMockReader(
+            columns: new[] { "Value" },
+            values: new object[] { "123-456" });
+
+        var transformer = new RegexTransformer(
+            columnName: "Value",
+            pattern: @"(\d+)-(\d+)",
+            replacement: "$2-$1");
+
+        // Act
+        var reader = transformer.Transform(source);
+        source.Read().Returns(true);
+        reader.Read();
+
+        // Assert
+        Assert.Equal("456-123", reader.GetValue(0));
+    }
+
+    [Fact]
+    public void IgnoreCase_MatchesDifferentCase()
+    {
+        // Arrange
+        var source = CreateMockReader(
+            columns: new[] { "BatchID" },
+            values: new object[] { "IIS_12345" });
+
+        var transformer = new RegexTransformer(
+            columnName: "BatchID",
+            pattern: "^iis_", // lowercase pattern
+            replacement: "",
+            ignoreCase: true);
+
+        // Act
+        var reader = transformer.Transform(source);
+        source.Read().Returns(true);
+        reader.Read();
+
+        // Assert
+        Assert.Equal("12345", reader.GetValue(0));
+    }
+
+    [Fact]
+    public void NullValue_PassesThrough()
+    {
+        // Arrange
+        var source = CreateMockReader(
+            columns: new[] { "BatchID" },
+            values: new object[] { DBNull.Value });
+
+        var transformer = new RegexTransformer(
+            columnName: "BatchID",
+            pattern: "^IIS_",
+            replacement: "");
+
+        // Act
+        var reader = transformer.Transform(source);
+        source.Read().Returns(true);
+        reader.Read();
+
+        // Assert
+        Assert.Equal(DBNull.Value, reader.GetValue(0));
+    }
+
+    [Fact]
+    public void NonTargetColumn_Unchanged()
+    {
+        // Arrange
+        var source = CreateMockReader(
+            columns: new[] { "BatchID", "OtherColumn" },
+            values: new object[] { "IIS_12345", "IIS_Should_Not_Change" });
+
+        var transformer = new RegexTransformer(
+            columnName: "BatchID",
+            pattern: "^IIS_",
+            replacement: "");
+
+        // Act
+        var reader = transformer.Transform(source);
+        source.Read().Returns(true);
+        reader.Read();
+
+        // Assert
+        Assert.Equal("12345", reader.GetValue(0));
+        Assert.Equal("IIS_Should_Not_Change", reader.GetValue(1));
+    }
+
+    [Fact]
+    public void InvalidRegex_ThrowsOnTransform()
+    {
+        // Arrange
+        var source = CreateMockReader(
+            columns: new[] { "Value" },
+            values: new object[] { "test" });
+
+        var transformer = new RegexTransformer(
+            columnName: "Value",
+            pattern: "[invalid(regex",
+            replacement: "");
+
+        // Act & Assert
+        var ex = Assert.Throws<RegexParseException>(() => transformer.Transform(source));
+        Assert.Contains("Invalid", ex.Message, StringComparison.OrdinalIgnoreCase);
+    }
+
+    [Fact]
+    public void ColumnNotFound_ThrowsOnTransform()
+    {
+        // Arrange
+        var source = CreateMockReader(
+            columns: new[] { "Value" },
+            values: new object[] { "test" });
+        source.GetOrdinal("NonExistent").Returns(_ => throw new IndexOutOfRangeException("Column not found"));
+
+        var transformer = new RegexTransformer(
+            columnName: "NonExistent",
+            pattern: "test",
+            replacement: "");
+
+        // Act & Assert
+        Assert.Throws<IndexOutOfRangeException>(() => transformer.Transform(source));
+    }
+
     private static IDataReader CreateMockReader(string[] columns, object[] values)
     {
         var reader = Substitute.For<IDataReader>();