Fix regex

Author: dandavison

temporalio/nexus/_link_conversion.py

@@ -152,4 +152,4 @@ def _event_type_pascal_case_to_constant_case(s: str) -> str:
     >>> _event_type_pascal_case_to_constant_case("NexusOperationScheduled")
     "NEXUS_OPERATION_SCHEDULED"
     """
-    return re.sub(r"([^\b])([A-Z])", lambda m: "_".join(m.groups()), s).upper()
+    return re.sub(r"([A-Z])", r"_\1", s).lstrip("_").upper()

tests/nexus/test_workflow_caller.py

@@ -1286,6 +1286,7 @@ def test_link_conversion_utilities():
         ("Ab", "AB"),
         ("AbCd", "AB_CD"),
         ("AbCddE", "AB_CDD_E"),
+        ("ContainsAOneLetterWord", "CONTAINS_A_ONE_LETTER_WORD"),
         ("NexusOperationScheduled", "NEXUS_OPERATION_SCHEDULED"),
     ]:
         assert p2c(p) == c