Python regex replacement not working with group

Question

Trying to do the following: string = "2" replace to = "No 2"

using below regex in python:

re.sub(r'(.*)', r'No. \g<1>', '2')

But it returns:

'No. 2No. '

Can you please help what's going wrong?

Note: using 3.8.5 python version

Answer 1

For me it works (maybe different python versions... I'm using 3.6.11 and it seems like 3.8's [which I assume OP is using] re works differently):

In [1]: import re

In [2]: re.sub(r'(.*)', r'No. \g<1>', '2')
Out[2]: 'No. 2'

But, I would suggest using a better regex:

In [3]: re.sub(r'(\d+)', r'No. \g<1>', '2')
Out[3]: 'No. 2'

This is better since it only catches the digits in the string:

In [4]: re.sub(r'(\d+)', r'No. \g<1>', '2 3 4 5 blabla')
Out[4]: 'No. 2 No. 3 No. 4 No. 5 blabla'

where your regex will just take the entire string:

In [5]: re.sub(r'(.*)', r'No. \g<1>', '2 3 4 5 blabla')
Out[5]: 'No. 2 3 4 5 blabla'