Raise unknown named character errors with the sntufn tag

mathics_scanner/escape_sequences.py

@@ -117,11 +117,11 @@ def parse_escape_sequence(
         if i == len(source_text):
             # Note: named characters do not have \n's in them. (Is this right)?
             # FIXME: decide what to do here.
-            raise NamedCharacterSyntaxError("Syntax", "sntufn", source_text[pos:])
+            raise NamedCharacterSyntaxError("sntufn", source_text[pos:])
 
         named_character = parse_named_character(source_text, pos, i)
         if named_character is None:
-            raise NamedCharacterSyntaxError("Syntax", "sntufn", source_text[pos:i])
+            raise NamedCharacterSyntaxError("sntufn", source_text[pos:i])
 
         result += named_character
         pos = i + 1

test/test_escape_sequences.py

@@ -57,6 +57,26 @@ def test_invalid_named_character_sequences():
             parse_escape_sequence(text, 1, is_in_string=False)
 
 
+@pytest.mark.parametrize(
+    ("text", "name"),
+    [
+        (r"\[Fake]", "Fake"),
+        (r"\[Alpha\]", "Alpha\\"),
+        (r"\[\Sigma]", "\\Sigma"),
+        (r"\[Theta", "Theta"),
+    ],
+)
+def test_invalid_named_character_message(text, name):
+    """The error carries the "sntufn" tag and the unknown name as its first argument,
+    which is what the Syntax::sntufn message template expects."""
+    with pytest.raises(NamedCharacterSyntaxError) as excinfo:
+        parse_escape_sequence(text, 1, is_in_string=False)
+    error = excinfo.value
+    assert error.name == "Syntax"
+    assert error.tag == "sntufn"
+    assert error.args[0] == name
+
+
 def test_invalid_number_encoding():
     for text in (
         # Octal