BUG: to_datetime(format='...%f') parses nanoseconds (#48820)

doc/source/whatsnew/v1.5.1.rst

@@ -76,6 +76,7 @@ Fixed regressions
 - Fixed regression in :meth:`DataFrame.plot` ignoring invalid ``colormap`` for ``kind="scatter"`` (:issue:`48726`)
 - Fixed performance regression in :func:`factorize` when ``na_sentinel`` is not ``None`` and ``sort=False`` (:issue:`48620`)
 - Fixed regression causing an ``AttributeError`` during warning emitted if the provided table name in :meth:`DataFrame.to_sql` and the table name actually used in the database do not match (:issue:`48733`)
+- Fixed regression in :func:`to_datetime` when ``arg`` was a date string with nanosecond and ``format`` contained ``%f`` would raise a ``ValueError`` (:issue:`48767`)
 - Fixed :meth:`.DataFrameGroupBy.size` not returning a Series when ``axis=1`` (:issue:`48738`)
 - Fixed Regression in :meth:`DataFrameGroupBy.apply` when user defined function is called on an empty dataframe (:issue:`47985`)
 

pandas/_libs/tslibs/strptime.pyx

@@ -347,7 +347,7 @@ def array_strptime(ndarray[object] values, str fmt, bint exact=True, errors='rai
 """
 TimeRE, _calc_julian_from_U_or_W are vendored
 from the standard library, see
-https://github.com/python/cpython/blob/master/Lib/_strptime.py
+https://github.com/python/cpython/blob/main/Lib/_strptime.py
 The original module-level docstring follows.
 
 Strptime-related classes and functions.
@@ -383,6 +383,9 @@ class TimeRE(_TimeRE):
         """
         self._Z = None
         super().__init__(locale_time=locale_time)
+        # GH 48767: Overrides for cpython's TimeRE
+        #  1) Parse up to nanos instead of micros
+        self.update({"f": r"(?P<f>[0-9]{1,9})"}),
 
     def __getitem__(self, key):
         if key == "Z":

pandas/tests/tools/test_to_datetime.py

@@ -2815,3 +2815,21 @@ def test_to_datetime_cache_coerce_50_lines_outofbounds(series_length):
 
     with pytest.raises(OutOfBoundsDatetime, match="Out of bounds nanosecond timestamp"):
         to_datetime(s, errors="raise", utc=True)
+
+
+def test_to_datetime_format_f_parse_nanos():
+    # GH 48767
+    timestamp = "15/02/2020 02:03:04.123456789"
+    timestamp_format = "%d/%m/%Y %H:%M:%S.%f"
+    result = to_datetime(timestamp, format=timestamp_format)
+    expected = Timestamp(
+        year=2020,
+        month=2,
+        day=15,
+        hour=2,
+        minute=3,
+        second=4,
+        microsecond=123456,
+        nanosecond=789,
+    )
+    assert result == expected