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Python正则表达式


RegEx或正则表达式是形成搜索模式的一系列字符。

RegEx可用于检查字符串是否包含指定的搜索模式。


正则表达式模块

Python有一个名为的内置程序包re,可用于处理正则表达式。

导入re模块:

import re

Python中的RegEx

导入re模块后,可以开始使用正则表达式:

搜索字符串以查看它是否以“ The”开头并以“ Spain”结尾:

import re

txt = "The rain in Spain"
x = re.search("^The.*Spain$", txt)
自己尝试»

正则表达式功能

re模块提供了一组函数,使我们可以在字符串中搜索匹配项:

Function Description
findall Returns a list containing all matches
search Returns a Match object if there is a match anywhere in the string
split Returns a list where the string has been split at each match
sub Replaces one or many matches with a string


元字符

元字符是具有特殊含义的字符:

Character Description Example
[] A set of characters "[a-m]"
\ Signals a special sequence (can also be used to escape special characters) "\d"
. Any character (except newline character) "he..o"
^ Starts with "^hello"
$ Ends with "world$"
* Zero or more occurrences "aix*"
+ One or more occurrences "aix+"
{} Exactly the specified number of occurrences "al{2}"
| Either or "falls|stays"
() Capture and group    

特殊序列

\以下列表中的字符之一是一个特殊的序列,其后有特殊含义:

Character Description Example
\A Returns a match if the specified characters are at the beginning of the string "\AThe"
\b Returns a match where the specified characters are at the beginning or at the end of a word
(the "r" in the beginning is making sure that the string is being treated as a "raw string")
r"\bain"
r"ain\b"
\B Returns a match where the specified characters are present, but NOT at the beginning (or at the end) of a word
(the "r" in the beginning is making sure that the string is being treated as a "raw string")
r"\Bain"
r"ain\B"
\d Returns a match where the string contains digits (numbers from 0-9) "\d"
\D Returns a match where the string DOES NOT contain digits "\D"
\s Returns a match where the string contains a white space character "\s"
\S Returns a match where the string DOES NOT contain a white space character "\S"
\w Returns a match where the string contains any word characters (characters from a to Z, digits from 0-9, and the underscore _ character) "\w"
\W Returns a match where the string DOES NOT contain any word characters "\W"
\Z Returns a match if the specified characters are at the end of the string "Spain\Z"

套装

集合是在一对方括号内的一组字符 [],具有特殊含义:

Set Description
[arn] Returns a match where one of the specified characters (a, r, or n) are present
[a-n] Returns a match for any lower case character, alphabetically between a and n
[^arn] Returns a match for any character EXCEPT a, r, and n
[0123] Returns a match where any of the specified digits (0, 1, 2, or 3) are present
[0-9] Returns a match for any digit between 0 and 9
[0-5][0-9] Returns a match for any two-digit numbers from 00 and 59
[a-zA-Z] Returns a match for any character alphabetically between a and z, lower case OR upper case
[+] In sets, +, *, ., |, (), $,{} has no special meaning, so [+] means: return a match for any + character in the string

 

findall()函数

findall()函数返回包含所有匹配项的列表。

打印所有匹配项的列表:

import re

txt = "The rain in Spain"
x = re.findall("ai", txt)
print(x)
自己尝试»

列表按找到顺序包含匹配项。

如果找不到匹配项,则返回一个空列表:

如果找不到匹配项,则返回一个空列表:

import re

txt = "The rain in Spain"
x = re.findall("Portugal", txt)
print(x)
自己尝试»

 

search()函数

search()函数在字符串中搜索匹配项,如果存在匹配项,则返回Match对象

如果有多个匹配项,则仅返回匹配项的第一个匹配项:

搜索字符串中的第一个空格字符:

import re

txt = "The rain in Spain"
x = re.search("\s", txt)

print("The first white-space character is located in position:", x.start())
自己尝试»

如果未找到匹配项,None则返回

进行搜索,返回不匹配的内容:

import re

txt = "The rain in Spain"
x = re.search("Portugal", txt)
print(x)
自己尝试»

 

split()函数

split()函数返回一个列表,其中每个匹配项都已将字符串分割开:

在每个空格字符处分割:

import re

txt = "The rain in Spain"
x = re.split("\s", txt)
print(x)
自己尝试»

您可以通过指定maxsplit 参数来控制出现次数

仅在第一次出现时才拆分字符串:

import re

txt = "The rain in Spain"
x = re.split("\s", txt, 1)
print(x)
自己尝试»

 

sub()函数

sub()函数将匹配项替换为您选择的文本:

将每个空格字符替换为数字9:

import re

txt = "The rain in Spain"
x = re.sub("\s", "9", txt)
print(x)
自己尝试»

您可以通过指定count 参数来控制替换次数

替换前两个事件:

import re

txt = "The rain in Spain"
x = re.sub("\s", "9", txt, 2)
print(x)
自己尝试»

 

匹配对象

匹配对象是包含有关搜索和结果信息的对象。

注意:如果没有匹配项,None则将返回,而不是匹配对象。

进行搜索以返回匹配对象:

import re

txt = "The rain in Spain"
x = re.search("ai", txt)
print(x) #this will print an object
自己尝试»

Match对象具有用于检索有关搜索信息和结果的属性和方法:

.span()返回一个元组,其中包含比赛的开始和结束位置。
.string返回传递给函数
.group()的字符串,返回匹配的字符串部分

打印第一个匹配项的位置(开始和结束位置)。

正则表达式查找以大写字母“ S”开头的所有单词:

import re

txt = "The rain in Spain"
x = re.search(r"\bS\w+", txt)
print(x.span())
自己尝试»

打印传递给函数的字符串:

import re

txt = "The rain in Spain"
x = re.search(r"\bS\w+", txt)
print(x.string)
自己尝试»

打印匹配的字符串部分。

正则表达式查找以大写字母“ S”开头的所有单词:

import re

txt = "The rain in Spain"
x = re.search(r"\bS\w+", txt)
print(x.group())
自己尝试»

注意:如果没有匹配项,None则将返回,而不是匹配对象。