BUG: bdate_range with cbh fails

[jbrockmendel]

Pandas version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

# Based on https://github.com/pandas-dev/pandas/issues/24555#issuecomment-450909512

START = pd.Timestamp(2009, 3, 13)
END1 = pd.Timestamp(2009, 3, 18)
END2 = pd.Timestamp(2009, 3, 19)

freq = 'CBH'
a = pd.bdate_range(START, END1, freq=freq, weekmask='Mon Wed Fri',
                   holidays=['2009-03-14'])  # <- raises

freq = 'cbh'
a = pd.bdate_range(START, END1, freq=freq, weekmask='Mon Wed Fri',
                   holidays=['2009-03-14'])  # <- still raises

Issue Description

IIUC we deprecated the upper-cast "h" and so now instead of "CBH" we want "cbh". But inside bdate_range we have a check for isinstance(freq, str) and freq.startswith("C"). I suspect that needs to be updated to check for lowercase "c"? cc @natmokval ?

Expected Behavior

N/A

Installed Versions

Replace this line with the output of pd.show_versions()

[nihalsiddiqui7]

Hi 👋,
I’d like to work on fixing this issue. I’ve read through the description and believe I can update bdate_range to properly handle lowercase 'cbh' frequencies while maintaining backward compatibility with 'CBH'.
Could I take this up as a contribution?

[natmokval]

Hi 👋, I’d like to work on fixing this issue. I’ve read through the description and believe I can update bdate_range to properly handle lowercase 'cbh' frequencies while maintaining backward compatibility with 'CBH'. Could I take this up as a contribution?

Hi @nihalsiddiqui7, we deprecated uppercase aliases for the class CustomBusinessHour, so "CBH" should raise.
This issue is related to a larger deprecation task #52064 that I worked on. I think I just missed this particular case.
If you do not mind, I will fix it.

[natmokval]

yes, I agree @jbrockmendel, it seems we need to update the check for a lowercase "c" in

pandas/pandas/core/indexes/datetimes.py

Line 1136 in ea75dd7

if isinstance(freq, str) and freq.startswith("C"):

I will do it.

[nihalsiddiqui7]

Hi 👋, I’d like to work on fixing this issue. I’ve read through the description and believe I can update bdate_range to properly handle lowercase 'cbh' frequencies while maintaining backward compatibility with 'CBH'. Could I take this up as a contribution?

Hi @nihalsiddiqui7, we deprecated uppercase aliases for the class CustomBusinessHour, so "CBH" should raise. This issue is related to a larger deprecation task #52064 that I worked on. I think I just missed this particular case. If you do not mind, I will fix it.

Of course, @natmokval. Please feel free to take it.