pandas - join words after specific word

Question

I have a pandas dataframe with a column of comments in the form

import pandas as pd
df = pd.DataFrame({'comment':['aaa bbb ccc not verb ddd']}) 
df.loc[0,'comment']

'aaa bbb ccc not verb ddd'

I want to join together the not with the word after it, in the example verb as not_verb , and return the rest of the row as is:

'aaa bbb ccc not_verb ddd'

Any help is appreciated.

EDIT:

Basically I'd like to join from " not " to the end of the word following it.

Answer 1

Use str.replace :

df.comment.str.replace(r'\b(not\s)', 'not_')

Output:

0    aaa bbb ccc not_verb ddd
Name: comment, dtype: object