Re regular expressions

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re正则表达式

正则表达式功能强大,But it's still more complicated,Referencing the official documentation is not very easy to write.Here is a brief record,To be proficient or to use more.After writing part of it, I found that it was a bit difficult to write,

概述

正则表达式（称为RE,或正则,or a regular expression pattern）Essentially embedded inPythona tiny one、高度专业化的编程语言.

Strings can be parsed and processed through regular expressions.Commonly used are matching,替换,分割等操作.

匹配

Matching characters are the most important part of a regular expression.

Most letters and characters will only match themselves.比如正则表达式testwill only match the string exactly'test'.注意：Without setting the relevant parameters,Regular expressions are strictly case-sensitive.

Except for normal characters,There are also some special characters in regular,These special characters are called metacharacters.以下是常用的元字符:

. ^ $ * + ? { } [ ] \ | ( )

These are introduced one by one below:

.：This character matches any character except newlines.

^：This character matches the beginning of the string.

$：匹配字符串的结尾

*：Matches the preceding regular expression0到任意次,And it's as many matches as possible.

+：Matches the preceding regular expression1到任意次,And it's as many matches as possible.

?：Matches the preceding regular expression0或1This repeats.

{m}：对其之前的正则式指定匹配 m 个重复;少于 m 的话就会导致匹配失败.

{m,n}：对正则式进行 m 到 n 次匹配,在 m 和 n 之间取尽量多.

{m,n}?：对正则式进行 m 到 n 次匹配,在 m 和 n Take as little as possible.

[...]：matches appear...中的字符.If you want to match a set of characters,They can be listed individually,也可以使用-to concatenate the start and end characters of the set of characters,For example to match all lowercase letters[a-z],从ASCIIlook at the code,[a-z]可以匹配a和z之间的所有字符.

[^...]：Match does not appear...中的字符

|：A|B,A和B可以是任意正则表达式,那么匹配A或者B.

()：组合.匹配括号内的任意正则表达式.After the matching is completed, the matching results in parentheses can be extracted.

There are also some special sequences here,如下:

\d \D \s \S \w \W

Introduce their functions：

\d

匹配任何十进制数字;这等价于 [0-9].

\D

匹配任何非数字字符;这等价于 [^0-9].

\s

匹配任何空白字符;这等价于 [ \t\n\r\f\v].

\S

匹配任何非空白字符;这相当于 [^ \t\n\r\f\v].

\w

匹配任何字母与数字字符;这相当于 [a-zA-Z0-9_].

\W

匹配任何非字母与数字字符;这相当于 [^a-zA-Z0-9_].

Backslash disaster

在Python的字符串中,字符\需要使用\\来标识.

Suppose you write a regex to match strings'\section'.Then you need to use regular\\\\section来表示.

in a regex that uses backslashes repeatedly,This results in a lot of repeated backslashes,and makes the resulting string incomprehensible.

使用正则表达式

编译正则表达式

Compiles a regular expression into a pattern object,In turn, various operations are performed through the schema object.

import re 
p = re.complie('ab*') #Compile the regular expression into a pattern object
print(type(p)) #<class 're.Pattern'>

res = p.match('abc')
print(type(res)) #<class 're.Match'>

应用匹配

Once you have an object representing the compiled regular expression,你用它做什么？ Schema objects have several methods and properties. Only the most important ones are covered here.

方法 / 属性	目的
match()	Determines whether the regex matches from the beginning of the string.In fact, it is to match the entire string with the regular expression.
search()	扫描字符串,Find anywhere this regex matches.
findall()	Find all substrings matched by the regular,and return them as a list.
finditer()	Find all substrings matched by the regular,and return them as one iterator.

• match():匹配整个字符串,返回Match

import re
p = re.compile('[a-z]+')
p.match(" ")  #None
m = p.match("tempo")
print(m) #<re.Match object; span=(0, 5), match='tempo'>

• search():Matches the entire string and its substrings,返回Match

p = re.compile('[a-z]+')
m = p.search("::: message");print(m)
# <re.Match object;span=(4,11), match='message'>

• findall():Returns a list of matching strings

p = re.compile(r'\d+')
m = p.findall('12 drummers drumming, 11 pipers piping, 10 lords a-leaping')
print(m)
# ['12', '11', '10']

• finditer():Returns a sequence of matching objects as oneiterator

iterator = p.finditer('12 drummers drumming, 11 ... 10 ...')
print(type(iterator))
# <class 'callable_iterator'>

The code above returnsre.Match有一些常用的方法.

方法/属性	目的
group()	Returns the regular matched string
start()	返回匹配的开始位置
end()	返回匹配的结束位置
span()	返回包含匹配 (start, end) 位置的元组

示例:

import re
p = re.compile('[a-z]+')
p.match(" ")  #None
m = p.match("tempo")
print(m) #<re.Match object; span=(0, 5), match='tempo'>


m.group() # 'tempo'
print(m.start(), m.end()) #0 5
print(m.span()) #(0, 5) 

分组

If you need to extract part of the matched string,Then you need to use grouping.

p = re.compile('(\w*)\s(\w*).*')
res = p.match('abc word hello')
print(res.group(0)) #abc word hello
print(res.group(1)) #abc
print(res.group(2)) #word

贪婪与非贪婪

This is to be introduced.*和.*?.

.*：尽可能多地匹配..*?：尽可能少地匹配.

s = '<html><head><title>Title</title>'
print(re.match('<.*>', s).group())
# <html><head><title>Title</title>
print(re.match('<.*?>', s).group())
# <html>

参考资料

• https://docs.python.org/zh-cn/3.8/howto/regex.html#match-versus-search
• https://docs.python.org/zh-cn/3.8/library/re.html#re-syntax
• https://blog.csdn.net/yuan2019035055/article/details/124217883