fix: report an invalid --skip-env regex as a handled error (#4029)

Co-authored-by: VXNCXNX <vxncxnx@users.noreply.github.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: VXNCXNX

docs/changelog/4029.bugfix.rst

@@ -0,0 +1,2 @@
+Report an invalid ``--skip-env``/``TOX_SKIP_ENV`` regular expression as a handled error instead of raising an unhandled
+``re.error`` traceback - by :user:`VXNCXNX`

src/tox/session/env_select.py

@@ -247,7 +247,15 @@ def __init__(self, state: State) -> None:
 
         self._state.conf.core.add_config("labels", dict[str, EnvList], {}, "core labels")
         tox_env_filter_regex = getattr(state.conf.options, "skip_env", "").strip()
-        self._filter_re = re.compile(tox_env_filter_regex) if tox_env_filter_regex else None
+        self._filter_re = self._compile_filter(tox_env_filter_regex) if tox_env_filter_regex else None
+
+    @staticmethod
+    def _compile_filter(pattern: str) -> re.Pattern[str]:
+        try:
+            return re.compile(pattern)
+        except re.error as exc:
+            msg = f"invalid environment skip filter {pattern!r}: {exc}"
+            raise HandledError(msg) from exc
 
     @property
     def _cli_envs(self) -> CliEnv | None:

tests/session/test_env_select.py

@@ -246,6 +246,22 @@ def test_tox_skip_env_logs(tox_project: ToxProjectCreator, monkeypatch: MonkeyPa
     outcome.assert_out_err("ROOT: skip environment mypy, matches filter 'm[y]py'\npy310\npy39\n", "")
 
 
+@pytest.mark.parametrize("bad_filter", ["[", "(", "*"])
+def test_tox_skip_env_invalid_regex(tox_project: ToxProjectCreator, monkeypatch: MonkeyPatch, bad_filter: str) -> None:
+    monkeypatch.delenv("TOX_SKIP_ENV", raising=False)
+    project = tox_project({"tox.ini": "[tox]\nenv_list = py3{10,9},mypy"})
+
+    leaked: BaseException | None = None
+    try:
+        outcome = project.run("l", "--no-desc", "--skip-env", bad_filter)
+    except Exception as exception:  # ruff:ignore[blind-except]
+        leaked = exception
+    assert leaked is None, f"unhandled {type(leaked).__name__}: {leaked}"
+
+    outcome.assert_failed()
+    assert f"HandledError| invalid environment skip filter {bad_filter!r}" in outcome.out
+
+
 def test_multiple_e_flags_are_additive(tox_project: ToxProjectCreator) -> None:
     proj = tox_project({"tox.ini": "[tox]\nenv_list=a,b,c"})
     outcome = proj.run("c", "-e", "a", "-e", "b", "-k", "env_name")