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A regular expression

1.1 Introduction

Regular expression is a special sequence of characters, which can easily check whether a string matches a certain pattern. The
re module makes the Python language have all the regular expression functions.

1.2 re.match function

re.match tries to match a pattern from the beginning of the string. If the match is not successful at the beginning, match() returns none.

函数语法：
re.match(pattern, string, flags=0)
函数参数说明
pattern	匹配的正则表达式
string	要匹配的字符串。
flags	标志位，用于控制正则表达式的匹配方式
匹配成功re.match方法返回一个匹配的对象，否则返回None

Instance

#!/usr/bin/python
#-*- coding: UTF-8 -*- 
import re
print(re.match('www', 'www.runoob.com').span())#在起始位置匹配
print(re.match('com', 'www.runoob.com'))#不在起始位置匹配

以上实例运行输出结果为：
(0, 3)
None

1.3 re.search method

re.search scans the entire string and returns the first successful match.

函数语法：
re.search(pattern, string, flags=0)
函数参数说明：
pattern	匹配的正则表达式
string	要匹配的字符串。
flags	标志位，用于控制正则表达式的匹配方式
匹配成功re.search方法返回一个匹配的对象，否则返回None

实例
#!/usr/bin/python
#-*- coding: UTF-8 -*- 
import re
print(re.search('www', 'www.runoob.com').span())#在起始位置匹配
print(re.search('com', 'www.runoob.com').span())#不在起始位置匹配

以上实例运行输出结果为：
(0, 3)
(11, 14)

1.4 Examples of regular expressions

1.4.1 Character Class

1. [0-9] matches any number. Similar to [0123456789]
2. [az] matches any lowercase letter
3. [AZ] matches any uppercase letter
4. [a-zA-Z0-9] matches any letter and number
5. [^aeiou] All characters except aeiou letters
6. [^0-9] matches characters except numbers

1.4.2 Special Character Class

1. Matches any single character except "\n". To match any character including'\n', please use a pattern like'[.\n]'
2. \d matches a digit character. Equivalent to [0-9]
3. \D matches a non-digit character. Equivalent to [^0-9]
4. \s matches any blank character, including spaces, tabs, form feeds, etc. Equivalent to [\f\n\r\t\v]
5. \S matches any non-whitespace character. Equivalent to [^ \f\n\r\t\v]
6. \w matches any word character including underscore. Equivalent to'[A-Za-z0-9_]'
7. \W matches any non-word character. Equivalent to'[^A-Za-z0-9_]'