Add failing reproduction test case

tests/test_regressions.py

@@ -1,9 +1,12 @@
 """Miscellaneous regression tests"""
 import pytest
 
+from httpie.cli.argtypes import KeyValueArgType
+from httpie.cli.constants import SEPARATOR_HEADER, SEPARATOR_QUERY_PARAM, SEPARATOR_DATA_STRING
+from httpie.cli.requestitems import RequestItems
 from httpie.compat import is_windows
-from .utils.matching import assert_output_matches, Expect
 from .utils import HTTP_OK, MockEnvironment, http
+from .utils.matching import assert_output_matches, Expect
 
 
 def test_Host_header_overwrite(httpbin):
@@ -47,3 +50,21 @@ def test_verbose_redirected_stdout_separator(httpbin):
         Expect.RESPONSE_HEADERS,
         Expect.BODY,
     ])
+
+
+@pytest.mark.parametrize(['separator', 'target'], [
+    (SEPARATOR_HEADER, 'headers'),
+    (SEPARATOR_QUERY_PARAM, 'params'),
+    (SEPARATOR_DATA_STRING, 'data'),
+])
+def test_initial_backslash_number(separator, target):
+    """
+    <https://github.com/httpie/httpie/issues/1408>
+    """
+    back_digit = r'\0'
+    raw_arg = back_digit + separator + back_digit
+    expected_parsed_data = {back_digit: back_digit}
+    parsed_arg = KeyValueArgType(separator)(raw_arg)
+    items = RequestItems.from_args([parsed_arg])
+    parsed_data = getattr(items, target)
+    assert parsed_data == expected_parsed_data