Extract part of string in Pandas column to a new column

I have a simple Pandas dataframe in Python consisting of a few columns like in the example below:

df_test = pd.DataFrame(data = None, columns = ['file','comment'])
df_test.file = ['file_1', 'file_1_v2', 'file_2', 'file_2_v2', 'file_3', 'file_3_v2']
df_test.comment = ['none: 5', 'Replacing: file_1', 'none', 'Replacing: file_2', 'none', 'Replacing: file_3']

What I want to do is to create a new column that combines strings from the other ones in the following manner:

The new column should contain the second part of the string in the 'comment' column if that string starts with 'Replacing: '.

If the 'comment' column does not start with this string, it should instead fill it with the value of 'file' in that position.

The end result for this example should be a column with the strings

['file_1', 'file_1', 'file_2', 'file_2', 'file_3', 'file_3']

It would be pretty easy if no other entries in 'comment' contained a colon than the ones that should be used, but like I entered in the example some of them do, meaning something like

df_test['comment'].str.extract(r'\s(.*)$', expand=False).fillna(df_test['file'])

will not work, as this one would split the string along every colon, which should not be the case. Any help is appreciated, thanks!

Use Replacing: (.*) as regex to force matching the "Replacing: ", the non-matches will be NaN:

df_test['comment'].str.extract(r'Replacing: (.*)', expand=False).fillna(df_test['file'])

Output:

0    file_1
1    file_1
2    file_2
3    file_2
4    file_3
5    file_3
Name: comment, dtype: object