BUG: pandas.tseries.frequencies.to_offset() raises ValueError when parsing a LastWeekOfMonth frequency string (#59245)

* add LastWeekOfMonth to prefix_mapping

* update whatsnew

Author: aram-cedarwood

doc/source/whatsnew/v3.0.0.rst

@@ -503,6 +503,7 @@ Datetimelike
 - Bug in :func:`date_range` where the last valid timestamp would sometimes not be produced (:issue:`56134`)
 - Bug in :func:`date_range` where using a negative frequency value would not include all points between the start and end values (:issue:`56382`)
 - Bug in :func:`tseries.api.guess_datetime_format` would fail to infer time format when "%Y" == "%H%M" (:issue:`57452`)
+- Bug in :func:`tseries.frequencies.to_offset` would fail to parse frequency strings starting with "LWOM" (:issue:`59218`)
 - Bug in :meth:`Dataframe.agg` with df with missing values resulting in IndexError (:issue:`58810`)
 - Bug in :meth:`DatetimeIndex.is_year_start` and :meth:`DatetimeIndex.is_quarter_start` does not raise on Custom business days frequencies bigger then "1C" (:issue:`58664`)
 - Bug in :meth:`DatetimeIndex.is_year_start` and :meth:`DatetimeIndex.is_quarter_start` returning ``False`` on double-digit frequencies (:issue:`58523`)

pandas/_libs/tslibs/offsets.pyx

@@ -4676,6 +4676,7 @@ prefix_mapping = {
         Hour,  # 'h'
         Day,  # 'D'
         WeekOfMonth,  # 'WOM'
+        LastWeekOfMonth,  # 'LWOM'
         FY5253,
         FY5253Quarter,
     ]

pandas/tests/tslibs/test_to_offset.py

@@ -31,6 +31,7 @@
         ("2SME-16", offsets.SemiMonthEnd(2, day_of_month=16)),
         ("2SMS-14", offsets.SemiMonthBegin(2, day_of_month=14)),
         ("2SMS-15", offsets.SemiMonthBegin(2)),
+        ("LWOM-MON", offsets.LastWeekOfMonth()),
     ],
 )
 def test_to_offset(freq_input, expected):