randomize each word in a string except the first and last letters

randomize.py

import random
import re

def word_stream(sentence):
    buff = ''
    for c in sentence:
        if 'A' <= c <= 'Z' or 'a' <= c <= 'z':
            buff += c
        else:
            if buff:
                yield buff
                buff = ''
            yield c

    if buff:
        yield buff

def scramble(word):
    if word is None or len(word) < 4:
        return word

    sub = list(word[1:-1])
    random.shuffle(sub)
    return word[0] + ''.join(sub) + word[-1]

def main():
    from sys import stdout
    sentence = raw_input('Enter a sentence: ')
    for match in word_stream(sentence):
        stdout.write(scramble(match))
    stdout.write('\n')

if __name__ == '__main__':
    main()

Example input: i went to the zoo on friday where my friends and i saw many zebras, monkeys, cats and elephants

Example output: i wnet to the zoo on fiadry werhe my feidnrs and i saw many zearbs, mkoynes, ctas and eelnthpas