正则表达式就是定义一个字符串的匹配模式。
与正则表达式相关的文档如下:

例子1：假如我们想要得到6-20位字符串并且该字符串由数字字母下划线构成。
在没有使用正则表达式需要写如下的代码：

def is_true(num):
    if 6 <= len(num) <= 20:
        for x in num:
            if not ('0' <= x <= '9' or 'A' <= x <= 'Z'
                    or 'a' <= x <= 'z' or x == '_'):
                return False
        return True
    return False

在使用正则表达式之后：

    username = input('username = ')
    pattern1 = re.compile(r'^[0-9a-zA-Z_]{6,20}$')
    m1 = pattern1.match(username)

下面就是一个完整使用正则表达式的例子：

import re


def main():
    username = input('username = ')
    qqnum = input('qqnum = ')
    pattern1 = re.compile(r'^[0-9a-zA-Z_]{6,20}$')
    m1 = pattern1.match(username)
    if not m1:
        print('用户格式错误')
    m2 = re.match(r'^[1-9]\d{4,11}$', qqnum)  # 在使用r的时候表示字符串为原始的字符串，如果不使用r那么在这里的正则表达式就该写成:'^[1-9]\\d{4,11}$'
    if not m2:
        print('QQ号格式错误')
    if m1 and m2:
        print('匹配成功')

if __name__ == '__main__':
    main()

map函数的使用：

from re import findall


def main():
    content = 'dsada4564sa4ds6a4d6sa'
    mylist = findall(r'\d+', content)
    mylist = list(map(int, mylist))
    print(sum(mylist) / len(mylist))  # 916.8


if __name__ == '__main__':
    main()

例子

1.手机号的验证

import re


def main():
    pattern = re.compile(r'(?<=\D)1([38][0-9]|[5][0-35-9]|[4][57]|[7][678])\d{8}(?=\D)')
    sentence = '大1558454564564架上15502344653读看18883362572洒金18883362572ljl13402311456ll'
    for temp in pattern.finditer(sentence):
        print(temp)  # <_sre.SRE_Match object; span=(16, 27), match='15502344653'>
        print(temp.group())  # 15502344653
        print(temp.span())  # (16, 27)


if __name__ == '__main__':
    main()

2.替换字符串中的不良文件：

import re


def main():
    sentence = '我操你大爷的日你二爷干你三舅姥爷Fuck你姑父'
    pure = re.sub('[操艹草日干顶妈]|fuck|shit', '*',
                  sentence, flags=re.IGNORECASE)
    print(pure)
    sentence = 'Your go your way, I will go mine!'  # 会将 , !.改成空格字符串
    mylist = re.split(r'[\s,!.]', sentence)
    print(mylist)

    # 贪婪匹配
    sentence = 'aabahdhsjb'
    m = re.match(r'a.*b', sentence)
    print(m)
    # 懒惰匹配
    sentence = 'aabahdhsjb'
    m = re.match(r'a.*?b', sentence)
    print(m)


if __name__ == '__main__':
    main()

3.得到字符串中的数字求平均值：

import re


def avage(num):
    pattern = re.compile(r'\d+')
    m = pattern.findall(num)
    totle = 0
    i = 0
    for x in m:
        totle += int(x)
        i += 1
    return totle / i


def main():
    n = input('n = ')
    print(avage(n))


if __name__ == '__main__':
    main()