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.
Python provides a built-in package named re that can be used to process regular expressions.
Import the re module:
import re
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")
The re module provides a set of functions that allow us to retrieve strings for matching:
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
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") 
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")
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")
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")
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")
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")
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")
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）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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