Return the actual symbols in the reversed range error

regexp_test.go

@@ -1241,3 +1241,14 @@ func TestConcatAccidentalPatternCharge(t *testing.T) {
 		t.Fatal("Expected non-nil, got nil")
 	}
 }
+
+func TestGoodReverseOrderMessage(t *testing.T) {
+	_, err := Compile(`[h-c]`, ECMAScript)
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	expected := "error parsing regexp: [h-c] range in reverse order in `[h-c]`"
+	if err.Error() != expected {
+		t.Fatalf("expected %q got %q", expected, err.Error())
+	}
+}

syntax/parser.go

@@ -104,7 +104,7 @@ const (
 	ErrBadClassInCharRange        = "cannot include class \\%v in character range"
 	ErrUnterminatedBracket        = "unterminated [] set"
 	ErrSubtractionMustBeLast      = "a subtraction must be the last element in a character class"
-	ErrReversedCharRange          = "[x-y] range in reverse order"
+	ErrReversedCharRange          = "[%c-%c] range in reverse order"
 )
 
 func (e ErrorCode) String() string {
@@ -1571,7 +1571,7 @@ func (p *parser) scanCharSet(caseInsensitive, scanOnly bool) (*CharSet, error) {
 				} else {
 					// a regular range, like a-z
 					if chPrev > ch {
-						return nil, p.getErr(ErrReversedCharRange)
+						return nil, p.getErr(ErrReversedCharRange, chPrev, ch)
 					}
 					cc.addRange(chPrev, ch)
 				}