import random

import re

def replace(t):
    inner_word = list(t.group(2))
    random.shuffle(inner_word)
    return t.group(1) + "".join(inner_word) + t.group(3)
text = "Hello, You should reach the finish line."
print re.sub(r"(\w)(\w+)(\w)", replace, text)

print re.sub(r"(\w)(\w+)(\w)", replace, text)

Hlleo, You slouhd raech the fsiinh lnie.
Hlleo, You suolhd raceh the fniish line.