regexp_replacer.py

import re

replacement_patterns = [
    (r'won\'t', 'will not'),
    (r'can\'t', 'can not'),
    (r'i\'m', 'i am'),
    (r'ain\'t', 'is not'),
    (r'(\w+)\'ll', '\g<1> will'),
    (r'(\w+)n\'t', '\g<1> not'),
    (r'(\w+)\'ve', '\g<1> have'),
    (r'(\w+)\'s', '\g<1> is'),
    (r'(\w+)\'re', '\g<1> are'),
    (r'(\w+)\'d', '\g<1> would')
]


class RegexpReplacer(object):
    def __init__(self, patterns=replacement_patterns):
        self.patterns = [(re.compile(regex), repl) for (regex, repl) in
                         patterns]

    def replace(self, text):
        s = text
        for (pattern, repl) in self.patterns:
            s = re.sub(pattern, repl, s)
        return s


replacer = RegexpReplacer()
print(replacer.replace("can't tokenize this"))