【Python入门第二十九天】Python RegEx

RegEx 或正则表达式是形成搜索模式的字符序列。

RegEx 可用于检查字符串是否包含指定的搜索模式。

RegEx 模块

Python 提供名为 re 的内置包，可用于处理正则表达式。

导入 re 模块：

import re
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Python 中的 RegEx

导入 re 模块后，就可以开始使用正则表达式了：

实例

检索字符串以查看它是否以 “China” 开头并以 “country” 结尾：

import re

txt = "China is a great country"
x = re.search("^China.*country$", txt)
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运行实例

import re

txt = "China is a great country"
x = re.search("^China.*country$", txt)

if (x):
  print("YES! We have a match!")
else:
  print("No match")
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RegEx 函数

re 模块提供了一组函数，允许我们检索字符串以进行匹配：

元字符

元字符是具有特殊含义的字符

字符：[]
描述：一组字符
示例：“[a-m]”

import re

str = "The rain in Spain"

#Find all lower case characters alphabetically between "a" and "m":

x = re.findall("[a-m]", str)
print(x)
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运行示例

字符：
描述：示意特殊序列（也可用于转义特殊字符）
示例：“\d”

import re

str = "That will be 59 dollars"

#Find all digit characters:

x = re.findall("\d", str)
print(x)

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运行示例

字符：.
描述：任何字符（换行符除外）
示例： “he…o”

import re

str = "hello world"

#Search for a sequence that starts with "he", followed by two (any) characters, and an "o":

x = re.findall("he..o", str)
print(x)
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运行示例

字符：^
描述：起始于
示例： “^hello”

import re

str = "hello world"

#Check if the string starts with 'hello':

x = re.findall("^hello", str)
if (x):
  print("Yes, the string starts with 'hello'")
else:
  print("No match")
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运行示例

字符：$
描述：结束于
示例：“world$”

import re

str = "hello world"

#Check if the string ends with 'world':

x = re.findall("world$", str)
if (x):
  print("Yes, the string ends with 'world'")
else:
  print("No match")
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运行示例

字符：*
描述：零次或多次出现
示例：“aix*”

import re

str = "The rain in Spain falls mainly in the plain!"

#Check if the string contains "ai" followed by 0 or more "x" characters:

x = re.findall("aix*", str)

print(x)

if (x):
  print("Yes, there is at least one match!")
else:
  print("No match")
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运行示例

字符：+
描述：一次或多次出现
示例： “aix+”

import re

str = "The rain in Spain falls mainly in the plain!"

#Check if the string contains "ai" followed by 1 or more "x" characters:

x = re.findall("aix+", str)

print(x)

if (x):
  print("Yes, there is at least one match!")
else:
  print("No match")
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运行示例

字符：{}
描述： 确切地指定的出现次数
示例：“al{2}”

import re

str = "The rain in Spain falls mainly in the plain!"

#Check if the string contains "a" followed by exactly two "l" characters:

x = re.findall("al{2}", str)

print(x)

if (x):
  print("Yes, there is at least one match!")
else:
  print("No match")
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运行示例

字符：|
描述：两者任一
示例：“falls|stays”

import re

str = "The rain in Spain falls mainly in the plain!"

#Check if the string contains either "falls" or "stays":

x = re.findall("falls|stays", str)

print(x)

if (x):
  print("Yes, there is at least one match!")
else:
  print("No match")
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运行示例

字符：()
描述：捕获和分组

特殊序列

特殊序列指的是 \ 后跟下表中的某个字符，拥有特殊含义。

字符：\A
描述：如果指定的字符位于字符串的开头，则返回匹配项
示例：“\AThe”

import re

str = "The rain in Spain"

#Check if the string starts with "The":

x = re.findall("\AThe", str)

print(x)

if (x):
  print("Yes, there is a match!")
else:
  print("No match")
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运行示例

字符：\b
描述：返回指定字符位于单词的开头或末尾的匹配项
示例：r"\bain"

import re

str = "The rain in Spain"

#Check if "ain" is present at the beginning of a WORD:

x = re.findall(r"\bain", str)

print(x)

if (x):
  print("Yes, there is at least one match!")
else:
  print("No match")
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运行示例

示例：r"ain\b"

import re

str = "The rain in Spain"

#Check if "ain" is present at the end of a WORD:

x = re.findall(r"ain\b", str)

print(x)

if (x):
  print("Yes, there is at least one match!")
else:
  print("No match")
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运行示例

字符：\B
描述：返回指定字符存在的匹配项，但不在单词的开头（或结尾处）
示例：r"\Bain"

import re

str = "The rain in Spain"

#Check if "ain" is present, but NOT at the beginning of a word:

x = re.findall(r"\Bain", str)

print(x)

if (x):
  print("Yes, there is at least one match!")
else:
  print("No match")

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运行示例

示例：r"ain\B"

import re

str = "The rain in Spain"

#Check if "ain" is present, but NOT at the end of a word:

x = re.findall(r"ain\B", str)

print(x)

if (x):
  print("Yes, there is at least one match!")
else:
  print("No match")
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运行示例

字符：\d
描述：返回字符串包含数字的匹配项（数字 0-9）
示例：“\d”

import re

str = "The rain in Spain"

#Check if the string contains any digits (numbers from 0-9):

x = re.findall("\d", str)

print(x)

if (x):
  print("Yes, there is at least one match!")
else:
  print("No match")
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运行示例

字符：\D
描述：返回字符串不包含数字的匹配项
示例：“\D”

import re

str = "The rain in Spain"

#Return a match at every no-digit character:

x = re.findall("\D", str)

print(x)

if (x):
  print("Yes, there is at least one match!")
else:
  print("No match")
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运行示例

字符：\s
描述：返回字符串包含空白字符的匹配项
示例：“\s”

import re

str = "The rain in Spain"

#Return a match at every white-space character:

x = re.findall("\s", str)

print(x)

if (x):
  print("Yes, there is at least one match!")
else:
  print("No match")

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运行示例

字符：\S
描述：返回字符串不包含空白字符的匹配项
示例：“\S”

import re

str = "The rain in Spain"

#Return a match at every NON white-space character:

x = re.findall("\S", str)

print(x)

if (x):
  print("Yes, there is at least one match!")
else:
  print("No match")
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运行示例

字符：\w
描述： 返回一个匹配项，其中字符串包含任何单词字符
（从 a 到 Z 的字符，从 0 到 9 的数字和下划线 _ 字符）
示例：“\w”

import re

str = "The rain in Spain"

#Return a match at every word character (characters from a to Z, digits from 0-9, and the underscore _ character):

x = re.findall("\w", str)

print(x)

if (x):
  print("Yes, there is at least one match!")
else:
  print("No match")
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运行示例

字符：\W
描述：返回一个匹配项，其中字符串不包含任何单词字符
示例：“\W”

import re

str = "The rain in Spain"

#Return a match at every NON word character (characters NOT between a and Z. Like "!", "?" white-space etc.):

x = re.findall("\W", str)

print(x)

if (x):
  print("Yes, there is at least one match!")
else:
  print("No match")
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运行示例

字符：\Z
描述：如果指定的字符位于字符串的末尾，则返回匹配项 。
示例：“Spain\Z”

import re

str = "The rain in Spain"

#Check if the string ends with "Spain":

x = re.findall("Spain\Z", str)

print(x)

if (x):
  print("Yes, there is a match!")
else:
  print("No match")
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运行示例

集合（Set）

集合（Set）是一对方括号 [] 内的一组字符，具有特殊含义。

字符：[arn]

描述：返回一个匹配项，其中存在指定字符（a，r 或 n）之一

示例

import re

str = "The rain in Spain"

#Check if the string has any a, r, or n characters:

x = re.findall("[arn]", str)

print(x)

if (x):
  print("Yes, there is at least one match!")
else:
  print("No match")
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运行示例

字符：[a-n]

描述：返回字母顺序 a 和 n 之间的任意小写字符匹配项

示例

import re

str = "The rain in Spain"

#Check if the string has any characters between a and n:

x = re.findall("[a-n]", str)

print(x)

if (x):
  print("Yes, there is at least one match!")
else:
  print("No match")
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运行示例

字符：[^arn]

描述：返回除 a、r 和 n 之外的任意字符的匹配项

示例

import re

str = "The rain in Spain"

#Check if the string has other characters than a, r, or n:

x = re.findall("[^arn]", str)

print(x)

if (x):
  print("Yes, there is at least one match!")
else:
  print("No match")
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运行示例

字符：[0123]

描述：返回存在任何指定数字（0、1、2 或 3）的匹配项

示例

import re

str = "The rain in Spain"

#Check if the string has any 0, 1, 2, or 3 digits:

x = re.findall("[0123]", str)

print(x)

if (x):
  print("Yes, there is at least one match!")
else:
  print("No match")
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运行示例

字符：[0-9]

描述：返回 0 与 9 之间任意数字的匹配

示例

import re

str = "8 times before 11:45 AM"

#Check if the string has any digits:

x = re.findall("[0-9]", str)

print(x)

if (x):
  print("Yes, there is at least one match!")
else:
  print("No match")
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运行示例

字符：[0-5][0-9]

描述：返回介于 0 到 9 之间的任何数字的匹配项

示例

import re

str = "8 times before 11:45 AM"

#Check if the string has any two-digit numbers, from 00 to 59:

x = re.findall("[0-5][0-9]", str)

print(x)

if (x):
  print("Yes, there is at least one match!")
else:
  print("No match")
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运行示例

字符：[a-zA-Z]

描述：返回字母顺序 a 和 z 之间的任何字符的匹配，小写或大写

示例

import re

str = "8 times before 11:45 AM"

#Check if the string has any characters from a to z lower case, and A to Z upper case:

x = re.findall("[a-zA-Z]", str)

print(x)

if (x):
  print("Yes, there is at least one match!")
else:
  print("No match")
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运行示例

字符：[+]

描述：在集合中，+、*、.、|、()、$、{} 没有特殊含义，因此 [+] 表示：返回字符串中任何 + 字符的匹配项。

示例

import re

str = "8 times before 11:45 AM"

#Check if the string has any + characters:

x = re.findall("[+]", str)

print(x)

if (x):
  print("Yes, there is at least one match!")
else:
  print("No match")
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运行示例

findall() 函数

findall() 函数返回包含所有匹配项的列表。

实例

打印所有匹配的列表

import re

str = "China is a great country"
x = re.findall("a", str)
print(x)
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运行实例

这个列表以被找到的顺序包含匹配项。

如果未找到匹配项，则返回空列表。

实例

如果未找到匹配，则返回空列表：

import re

str = "China is a great country"
x = re.findall("USA", str)
print(x)
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运行实例

search() 函数

search() 函数搜索字符串中的匹配项，如果存在匹配则返回 Match 对象。

如果有多个匹配，则仅返回首个匹配项。

实例

在字符串中搜索第一个空白字符

import re

str = "China is a great country"
x = re.search("\s", str)

print("The first white-space character is located in position:", x.start())
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运行实例

如果未找到匹配，则返回值 None：

实例

进行不返回匹配的检索

import re

str = "China is a great country"
x = re.search("USA", str)
print(x)
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运行实例

split() 函数

split() 函数返回一个列表，其中字符串在每次匹配时被拆分。

实例

在每个空白字符处进行拆分

import re

str = "China is a great country"
x = re.split("\s", str)
print(x)
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运行实例

可以通过指定 maxsplit 参数来控制出现次数：

实例

仅在首次出现时拆分字符串：

import re

str = "China is a great country"
x = re.split("\s", str, 1)
print(x)
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运行实例

sub() 函数

sub() 函数把匹配替换为您选择的文本

实例

用数字 9 替换每个空白字符

import re

str = "China is a great country"
x = re.sub("\s", "9", str)
print(x)
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运行实例

可以通过指定 count 参数来控制替换次数：

实例

替换前两次出现

import re

str = "China is a great country"
x = re.sub("\s", "9", str, 2)
print(x)
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运行实例

Match 对象

Match 对象是包含有关搜索和结果信息的对象。

注释：如果没有匹配，则返回值 None，而不是 Match 对象。

实例

执行会返回 Match 对象的搜索：

import re

str = "China is a great country"
x = re.search("a", str)
print(x) # 将打印一个对象
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运行实例

Match 对象提供了用于取回有关搜索及结果信息的属性和方法：

• span() 返回的元组包含了匹配的开始和结束位置
• .string 返回传入函数的字符串
• group() 返回匹配的字符串部分

实例

打印首个匹配出现的位置（开始和结束位置）。

正则表达式查找以大写 “C” 开头的任何单词：

import re

str = "China is a great country"
x = re.search(r"\bC\w+", str)
print(x.span())
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运行实例

实例

打印传入函数的字符串

import re

str = "China is a great country"
x = re.search(r"\bC\w+", str)
print(x.string)
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运行实例

实例

打印匹配的字符串部分

正则表达式查找以大写 “C” 开头的任何单词：

import re

str = "China is a great country"
x = re.search(r"\bC\w+", str)
print(x.group())
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运行实例

注释：如果没有匹配项，则返回值 None，而不是 Match 对象。

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