Use todo.txt spec for id-ing completed tasks

Topydo was only identifying a task as completed if it included a
completion date (which topydo automatically adds). This affects
compatibility with other todo.txt apps, making archiving unreliable.

test/test_parse.py

@@ -0,0 +1,68 @@
+# Topydo - A todo.txt client written in Python.
+# Copyright (C) 2023 David Steele <dsteele@gmail.com>
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+import unittest
+
+from topydo.lib.TodoParser import parse_line
+
+complete_tasks = [
+    "x 2023-08-12",
+    "x 2023-08-12 ",
+    "x 2023-08-12 2023-08-12",
+    "x 2023-08-12 2023-08-12 ",
+    "x 2023-08-12 2023-08-12 task text",
+    "x 2023-08-12 2023-08-12 task text",
+    "x 2023-08-12 task text",
+    "x task text",
+    "x ",
+]
+
+incomplete_tasks = [
+    "",
+    " ",
+    " x",
+    " x ",
+    "x",
+    "incomplete task",
+]
+
+
+def gen_test(task, result):
+    def test_fn(self):
+        task_dict = parse_line(task)
+
+        self.assertEqual(task_dict["completed"], result, f'Failed "{task}"')
+
+    return test_fn
+
+
+def populate_tasks(cls):
+    for i, task in enumerate(complete_tasks):
+        setattr(cls, f"test_complete_{i}", gen_test(task, True))
+
+    for i, task in enumerate(incomplete_tasks):
+        setattr(cls, f"test_incomplete_{i}", gen_test(task, False))
+
+    return cls
+
+
+@populate_tasks
+class ParseTodoTest(unittest.TestCase):
+    pass
+
+
+if __name__ == "__main__":
+    unittest.main()

topydo/lib/TodoParser.py

@@ -26,7 +26,7 @@ from topydo.lib.Utils import date_string_to_date
 _DATE_MATCH = r'\d{4}-\d{2}-\d{2}'
 
 _COMPLETED_HEAD_MATCH = re.compile(
-    r'x ((?P<completionDate>' + _DATE_MATCH + ') )' + '((?P<creationDate>' +
+    r'x ((?P<completionDate>' + _DATE_MATCH + ') )?' + '((?P<creationDate>' +
     _DATE_MATCH + ') )?(?P<rest>.*)')
 
 _NORMAL_HEAD_MATCH = re.compile(