Include regex compiler detail in CRegExp errors

tests/test_traitlets.py

@@ -1902,9 +1902,32 @@ def coerce(self, value):
     obj = CRegExpTrait()
 
     _default_value = re.compile(r"")
-    _good_values = [r"\d+", re.compile(r"\d+")]
+    _good_values = [r"\d+", re.compile(r"\d+"), rb"\d+"]
     _bad_values = ["(", None, ()]
 
+    def test_invalid_pattern_error_message(self):
+        pattern = "["
+        with pytest.raises(re.error) as regex_error:
+            re.compile(pattern)
+
+        with pytest.raises(TraitError) as trait_error:
+            self.assign(pattern)
+
+        assert str(regex_error.value) in str(trait_error.value)
+
+    def test_invalid_pattern_error_message_in_list(self):
+        pattern = "["
+        with pytest.raises(re.error) as regex_error:
+            re.compile(pattern)
+
+        class CRegExpListTrait(HasTraits):
+            values = List(CRegExp())
+
+        with pytest.raises(TraitError) as trait_error:
+            CRegExpListTrait(values=[pattern])
+
+        assert str(regex_error.value) in str(trait_error.value)
+
 
 class DictTrait(HasTraits):
     value = Dict()

traitlets/traitlets.py

@@ -4225,6 +4225,8 @@ class CRegExp(TraitType[re.Pattern[t.Any], re.Pattern[t.Any] | str]):
     def validate(self, obj: t.Any, value: t.Any) -> re.Pattern[t.Any] | None:
         try:
             return re.compile(value)
+        except re.error as e:
+            self.error(obj, value, info=f"{self.info()}: {e}")
         except Exception:
             self.error(obj, value)