BUG guess_Datetime_format doesn't guess 27.03.2003 14:55:00.000 (#50319)

* fix parsing

* stricter regex

Co-authored-by: MarcoGorelli <>

Author: MarcoGorelli

pandas/_libs/tslibs/parsing.pyx

@@ -1016,9 +1016,11 @@ def guess_datetime_format(dt_str: str, bint dayfirst=False) -> str | None:
 
 cdef str _fill_token(token: str, padding: int):
     cdef str token_filled
-    if "." not in token:
+    if re.search(r"\d+\.\d+", token) is None:
+        # For example: 98
         token_filled = token.zfill(padding)
     else:
+        # For example: 00.123
         seconds, nanoseconds = token.split(".")
         seconds = f"{int(seconds):02d}"
         # right-pad so we get nanoseconds, then only take

pandas/tests/tslibs/test_parsing.py

@@ -179,6 +179,7 @@ def test_parsers_month_freq(date_str, expected):
         ("2011-12-30 00:00:00.000000", "%Y-%m-%d %H:%M:%S.%f"),
         ("Tue 24 Aug 2021 01:30:48 AM", "%a %d %b %Y %H:%M:%S %p"),
         ("Tuesday 24 Aug 2021 01:30:48 AM", "%A %d %b %Y %H:%M:%S %p"),
+        ("27.03.2003 14:55:00.000", "%d.%m.%Y %H:%M:%S.%f"),  # GH50317
     ],
 )
 def test_guess_datetime_format_with_parseable_formats(string, fmt):