Pandas dataframe merge by function on column names
A simple concatenation will do 
pd.concat([df_A, df_B], join='outer')[['A', 'B']].copy().
or
'pd.concat([df_A, df_B], join='inner')
I say to dataframes.
df_A has columns A__a, B__b, C. (shape 5,3)
df_B has columns A_a, B_b, D. (shape 4,3)
How can I unify them (without having to iterate over all columns) to get one df with columns A,B ? (shape 9,2) - meaning A__a and A_a should be unified to the same column.
I need to use merge with applying the function lambda x: x.replace("_",""). Is it possible?