自学python爬虫（三）正则表达式

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一、什么是正则表达式

正则表达式是对字符串操作的一种逻辑公式，就是用事先定义好的一些特定字符，及这些特定字符的组合，组成一个“规则字符串”，这个“规则字符串”用来表达对字符串的一种过滤逻辑。（非Python独有，python中re模块实现）

二、常见的匹配模式

re.match
re.match尝试从字符串的起始位置匹配一个模式，如果不是起始位置匹配成功的话，match()就返回None。
最常规的匹配：

import re

content = "Hello 123 4567 World_This is a Regex Demo"
result = re.match('^Hello\s\d\d\d\s\d{4}\s\w{10}.*Demo$',content)
print(result)

import re

content = "Hello 123 4567 World_This is a Regex Demo"
print(len(content))
result = re.match('^Hello\s\d\d\d\s\d{4}\s\w{10}.*Demo$',content)
print(result)
print(result.group())
print(result.span())

泛匹配

import re

content = "Hello 123 4567 World_This is a Regex Demo"
result = re.match('^Hello.*Demo$',content)
print(result)
print(result.group())
print(result.span())

匹配目标

import re

content = "Hello 1234567 World_This is a Regex Demo"
result = re.match('^Hello\s(\d+)\sWorld.*Demo$',content)
print(result)
print(result.group(1))
print(result.span())

贪婪匹配

import re

content = "Hello 1234567 World_This is a Regex Demo"
result = re.match('^He.*(\d+).*Demo$',content)
print(result)
print(result.group(1))
print(result.span())

非贪婪匹配

import re

content = "Hello 1234567 World_This is a Regex Demo"
result = re.match('^He.*?(\d+).*Demo$',content)
print(result)
print(result.group(1))
print(result.span())

匹配模式

import re

content = '''Hello 1234567 World_This
is a Regex Demo'''
result = re.match('^He.*(\d+).*Demo$',content)
print(result)
print(result.group(1))
print(result.span())

import re

content = '''Hello 1234567 World_This
is a Regex Demo'''
result = re.match('^He.*(\d+).*Demo$',content,re.S)
print(result)
print(result.group(1))
print(result.span())

转义

import re

content = 'price is $5.00'
result = re.match('^price is \$5\.00',content)
print(result)

总结：尽量使用泛匹配、是用括号得到匹配目标、尽量使用非贪婪模式、有换行符就用re.S

re.search

re.search扫描整个字符串并返回第一个成功的匹配。

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import re

content = "Extra String Hello 123 4567 World_This is a Regex Demo"
result = re.match('Hello.*?(\d+).*?Demo$',content)
print(result)

import re

content = "Extra String Hello 123 4567 World_This is a Regex Demo"
result = re.search('Hello.*?(\d+).*?Demo$',content)
print(result)

总结：为匹配方便，能用search就不用match。
re.findall


re.sub

import re

content = "Extra String Hello 1234567 World_This is a Regex Demo"
conteent = re.sub('\d+','replacement',content)
print(conteent)

import re

content = "Extra String Hello 1234567 World_This is a Regex Demo"
content = re.sub('\d+','',content)
print(content)

import re

content = "Extra String Hello 1234567 World_This is a Regex Demo"
content = re.sub('(\d+)',r'\1 8910',content)
print(content)

re.compile
将正则表达式编译为正则对象，以便复用该匹配模式。

import re

content = "Hello 123 4567 World_This is a Regex Demo"
pattern = re.compile('^Hello\s\d\d\d\s\d{4}\s\w{10}.*Demo$')
result = re.match(pattern,content)
print(result)