How to use Python's RegEx regular expression

RegEx or regular expression is a sequence of characters that forms a search pattern.

RegEx can be used to check whether a string contains a specified search pattern.

RegEx module

Python provides a built-in package named re that can be used to process regular expressions.

Import the re module:

import re

RegEx in Python

After importing the re module, you can start using regular expressions:

Example

Retrieve a string to see if it starts with "China" and ends with "country":

import re

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

Running Example

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")

RegEx Functions

The re module provides a set of functions that allow us to retrieve strings for matching:

Metacharacters

Metacharacters are characters with special meaning

Characters: [] Description: A set of characters Example: "[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)

Run the example

Characters: Description: Indicates special sequences (can also be used to escape special characters) Example: "\d"

import re

str = "That will be 59 dollars"

#Find all digit characters:

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

Run the example

Characters: . Description: Any character (except newline) Example: "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)

Run the example

Characters: ^ Description: Start with example: "^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")

Run example

Characters: $ Description: End with example: "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")

Run Example

Characters: * Description: Zero or more occurrences Example: "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")

Run Example

Characters: Description: One or more occurrences of Example: "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")

Run the example

Characters :{} Description: Exactly specified number of occurrences Example: "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")

Running Example

Characters: | Description: Both Either example: "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")

Running Example

##Characters: () Description: Capturing and grouping

special sequences

Special sequences refer to \ followed by a character in the table below, which has special meaning.

Character: \A Description: Returns a match if the specified character is at the beginning of the string Example: "\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")

Run the example

Character: \b

Description: Returns matches where the specified character is at the beginning or end of the word

Example: 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")

Run Example

Example: 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")

Run the example

Characters: \B

Description: Returns matches where the specified character exists, but is not at the beginning (or end) of the word

Example: 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")

Run the example

Example: 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")

Run the example

Characters: \d

Description: Returns matches where the string contains numbers (digits 0-9)

Example: "\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")

Run the example

Character: \D

Description: Returns matches where the string does not contain digits

Example: "\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")

Run the example

Characters: \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")

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")

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")

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")

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")

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")

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")

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")

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")

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")

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")

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")

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")

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)

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)

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())

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)

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)

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)

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)

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)

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

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())

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)

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())

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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