Add regex command implementation

tests/contract/regex_validation_test.go

@@ -0,0 +1,45 @@
+package contract
+
+import (
+	"context"
+	"testing"
+
+	"github.com/rogeecn/renamer/internal/regex"
+)
+
+func TestRegexTemplateRejectsUndefinedGroup(t *testing.T) {
+	req := regex.NewRequest(t.TempDir())
+	req.Pattern = "^(\\w+)-(\\d+)"
+	req.Template = "@3"
+
+	_, _, err := regex.Preview(context.Background(), req, nil)
+	if err == nil {
+		t.Fatalf("expected error for undefined capture group")
+	}
+}
+
+func TestRegexPreviewHandlesInvalidPattern(t *testing.T) {
+	req := regex.NewRequest(t.TempDir())
+	req.Pattern = "(([" // invalid pattern
+	req.Template = "@1"
+
+	_, _, err := regex.Preview(context.Background(), req, nil)
+	if err == nil {
+		t.Fatalf("expected error for invalid pattern")
+	}
+}
+
+func TestRegexPreviewSkipsUnmatchedOptionalGroup(t *testing.T) {
+	req := regex.NewRequest(t.TempDir())
+	req.Pattern = "^(\\w+)(?:-(\\d+))?"
+	req.Template = "@1_@2"
+
+	summary, _, err := regex.Preview(context.Background(), req, nil)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	if summary.TotalCandidates != 0 {
+		t.Fatalf("expected no candidates without files, got %d", summary.TotalCandidates)
+	}
+}