perf: use isidentifier() instead of regex in _build_path

Author: sean2077

jsonpath/jsonpath.py

@@ -212,7 +212,8 @@ def _build_path(path: str, key) -> str:
         """
         if isinstance(key, int):
             return f"{path}[{key}]"
-        if JSONPath.REP_WORD_KEY.match(key):
+        # Fast check: if all chars are word chars (alphanumeric + underscore)
+        if key.isidentifier() or (key and key.replace("_", "a").isalnum()):
             return f"{path}.{key}"
         return f"{path}['{key}']"