diff --git a/py-polars/polars/utils/udfs.py b/py-polars/polars/utils/udfs.py
index 81db4393d7a59..ae4964ea1cf44 100644
--- a/py-polars/polars/utils/udfs.py
+++ b/py-polars/polars/utils/udfs.py
@@ -206,7 +206,7 @@ def _get_target_name(self, col: str, expression: str) -> str:
                 n = count()
                 while True:
                     name = f"srs{next(n)}"
-                    if not re.search(rf"\bs{name}\b", search_expr):
+                    if not re.search(rf"\b{name}\b", search_expr):
                         return name
 
         raise NotImplementedError(f"TODO: apply_target = {self._apply_target!r}")