How to search and replace with enclosed characters a text file?

Given a textile how can I replace all the tokens that have % at the beginning for []. For instance in the following text file:

Hi how are you? 
I %am %fine.
Thanks %and %you

How can I enclose all the characters with % with []:

Hi how are you? 
I [am] [fine].
Thanks [and] [you]

I tried to first filter the tokens and then replace them but maybe there is a more pythonic way:

with open('../file') as f:
    s = str(f.readlines())
    a_list = re.sub(r'(?<=\W)[$]\S*', s.replace('.',''))
    a_list= set(a_list)
    print(list(a_list))

Solution

You may use

re.sub(r'\B%(\w+)', r'[\1]', s)

See the regex demo

Details

\B - a non-word boundary, there must be start of string or a non-word char immediately to the left of the current location
% - a % char
(\w+) - Group 1: any 1 or more word chars (letters, digits or _). Replace with (\S+) to match 1 or more non-whitespace chars if necessary, but note \S also matches punctuation.

Python demo:

import re

s = "Hi how are you? \nI %am %fine.\nThanks %and %you"
result = re.sub(r"\B%(\w+)", r"[\1]", s)
print(result)