Pandas how can 'replace' work after 'loc'?

Question

I have tried many times, but seems the \'replace\' can NOT work well after use \'loc\'. For example I want to replace the \'conlumn_b\' with an regex for the row that the \'

Answer 1

I think you need filter in both sides:

m = df['conlumn_a'] == 'apple'
df.loc[m,'conlumn_b'] = df.loc[m,'conlumn_b'].astype(str).replace(r'^(11+)','XXX',regex=True)
print (df)
  conlumn_a conlumn_b
0     apple       123
1    banana        11
2     apple       XXX
3    orange        33