Fixes bug when loading user-provided msg-template.

doc/whatsnew/fragments/5636.bugfix

@@ -0,0 +1,3 @@
+Using custom braces in ``msg-template`` will now work properly.
+
+Closes #5636

pylint/reporters/text.py

@@ -176,7 +176,7 @@ def on_set_current_module(self, module: str, filepath: str | None) -> None:
         self._template = template
 
         # Check to see if all parameters in the template are attributes of the Message
-        arguments = re.findall(r"\{(.+?)(:.*)?\}", template)
+        arguments = re.findall(r"\{(\w+?)(:.*)?\}", template)
         for argument in arguments:
             if argument[0] not in MESSAGE_FIELDS:
                 warnings.warn(

tests/reporters/unittest_reporting.py

@@ -90,16 +90,12 @@ def test_template_option_non_existing(linter: PyLinter) -> None:
     """
     output = StringIO()
     linter.reporter.out = output
-    linter.config.msg_template = (
-        "{path}:{line}:{a_new_option}:({a_second_new_option:03d})"
-    )
+    linter.config.msg_template = "{path}:{line}:{categ}:({a_second_new_option:03d})"
     linter.open()
     with pytest.warns(UserWarning) as records:
         linter.set_current_module("my_mod")
         assert len(records) == 2
-        assert (
-            "Don't recognize the argument 'a_new_option'" in records[0].message.args[0]
-        )
+        assert "Don't recognize the argument 'categ'" in records[0].message.args[0]
     assert (
         "Don't recognize the argument 'a_second_new_option'"
         in records[1].message.args[0]
@@ -115,6 +111,23 @@ def test_template_option_non_existing(linter: PyLinter) -> None:
     assert out_lines[2] == "my_mod:2::()"
 
 
+def test_template_option_with_header(linter: PyLinter) -> None:
+    output = StringIO()
+    linter.reporter.out = output
+    linter.config.msg_template = '{{ "Category": "{category}" }}'
+    linter.open()
+    linter.set_current_module("my_mod")
+
+    linter.add_message("C0301", line=1, args=(1, 2))
+    linter.add_message(
+        "line-too-long", line=2, end_lineno=2, end_col_offset=4, args=(3, 4)
+    )
+
+    out_lines = output.getvalue().split("\n")
+    assert out_lines[1] == '{ "Category": "convention" }'
+    assert out_lines[2] == '{ "Category": "convention" }'
+
+
 def test_deprecation_set_output(recwarn: WarningsRecorder) -> None:
     """TODO remove in 3.0."""
     reporter = BaseReporter()