Regular expression and re module in python

What is a regular expression:

A rule to match strings

What can regular expressions do:

• A rule can be formulated:

• To confirm whether a certain string meets the rules

• Find content that meets the rules from a large section of string

• Program area:

• Login form registration form verification

• reptile

• Automated development: log analysis

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Regular expression syntax:

• Metacharacters

• Character group [], what can appear at the position of a character

• quantifier

• Special usages and phenomena

• The use of?: Followed by a? In the quantifier to cancel greedy matching, that is, the lazy mechanism

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character:

Metacharacters	Matched content
.	Match any character except newline
\w	Match letters or numbers or underscores
\s	Match any whitespace
\d	Match number
\n	Matches a newline
\t	Match a tab
\b	Match the end of a word
^	Match the beginning of the string
$	Match the end of the string
\W	Match non-letters or numbers or underscores
\D	Match non-numeric
\S	Match non-whitespace characters
a | b	Match character a or character b
()	Matches the expression in parentheses, also means a group
[…]	Match the characters in the character group
[^…]	Match all characters except the characters in the character group

quantifier:

quantifier	Instructions
*	Repeat zero or more times
+	Repeat one or more times
?	Repeat zero or one time
{n}	Repeat n times
{n,}	Repeat n or more times
{n,m}	Repeat n to m times

Graphic memory? * + Meaning:

. ^ $

Regular	Characters to be matched	Match result	Explanation
sea.	Haiyan Haijiao Haidong	Haiyan Haijiao Haidong	Match all "sea." Characters
^ Sea.	Haiyan Haijiao Haidong	Haiyan	Match "海." Only from the beginning
Sea. $	Haiyan Haijiao Haidong	Haidong	Only match the ending "海. $"

Several commonly used non-greedy matching patterns

• *? Repeat any number of times, but repeat as little as possible
• +? Repeat 1 or more times, but repeat as little as possible
• ?? Repeat 0 or 1 times, but repeat as little as possible
• {n, m}? Repeat n to m times, but repeat as little as possible
• {n,}? Repeat n times or more, but repeat as little as possible

Common methods under the re module:

• Matching method

• findall matches all items and returns a list

• search If there is a return value, return an object, no return value, return none, the returned object through the group to get the first match

• match

• replace

• sub returns the replaced result

• subn returns a tuple (replace the result, how many times it has been replaced)

• Cut

• split

• Advanced method:

• compile compiles a regular expression into a regular expression object,

• finditer returns an iterator to store matching results

Examples:

#findall方法
import re
ret = re.findall('\d','hello123python456')
print(ret)

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#search
ret = re.search('\d','hello123python456')
print(ret) #返回一个对象
print(ret.group()) 返回第一个匹配的结果

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#split
ret = re.split('\d+','alex40taibai35codegod21')
print(ret) #返回以数字切割后的列表

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#sub,subn 替换的方法区别
ret = re.sub('\d','H','codegod1ello')
print(ret)
ret2 = re.subn('\d+','Joke','123')
print(ret2) #返回一个元组，(替换后的值，替换几次)

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#compile
obj = re.compile('\d{3}')
ret = obj.search('abc123eeee')
print(ret.group())

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ret = re.finditer('\d','sf230f3f9r39')
print(ret) #返回<callable_iterator object at 0x00000206653E6130>
print(next(ret).group()) #查看第一个结果
print(next(ret).group()) #查看第二个结果
print([i.group() for i in ret]) #查看剩余的左右结果

note:

1. findall priority query

import re
ret = re.findall('www.(baidu|oldboy).com','www.oldboy.com')
print(tet) #返回['oldboy'] 这是因为findall会优先把匹配结果组里内容返回，如果想要匹配结果，取消权限即可
ret = re.findall('www.(?:baidu|oldboy).com','www.oldboy.com')
print(ret

2. Split priority query

ret = re.split("\d+","eva3egon4yuan")
print(ret) 
ret2 = re.split("(\d+)",'eva3egon4yuan')
print(ret2)

The result after adding () to the regular matching part is different. Those without () do not retain the matched items, but with () they can retain the matched items.