Regular expressions

One 、 Introduction to regular expressions

The role of regular expressions

Actually, there are four main steps for reptiles ：

1. Clear objectives ( Know where you're going to search... Or where you're going to search )
2. climb ( All the content of the website will be crawled down )
3. take ( Get rid of data that's not useful to us )
4. Processing data （ Store and use in the way we want ）

Regular expressions are often used for retrieval 、 Replace those that match a pattern ( The rules ) The text of .

Regular expression is a logical formula for string operation , It is to use some specific characters defined in advance 、 And the combination of these specific characters , Form a “ Rule string ”, This “ Rule string ” A filter logic used to express strings .

Purpose of using regular expressions

Whether the given string conforms to the filtering logic of regular expression （“ matching ”）;
Through regular expressions , Get the specific part we want from the text string （“ Filter ”）.

Two 、 Regular expression matching rules

####### Common characters

Predefined character set

quantifiers

Boundary matching

Logical grouping

3、 ... and 、re modular

Regular expressions use Escape special characters , So if we're going to use the original string , Just add one r Prefix ,

r'chuanzhiboke\t\.\tpython'

re Module general use steps

1. Use compile() Function to compile a regular expression as a string Pattern object

2. adopt Pattern Object provides a series of methods for matching text , Get a match , One Match object .

3. Finally using Match Object provides properties and methods to get information , Do other operations as needed

compile function

compile Function to compile regular expressions , Generate a Pattern object , Its general usage is as follows ：

import re
# Compile regular expressions into Pattern object
pattern = re.compile(r'\d+')

Pattern object

Regular expressions are compiled into Pattern object , You can use pattern A series of methods for text matching to find .

Pattern Some commonly used methods of object mainly include ：
match Method ： Find from start , One match
search Method ： Search from anywhere , One match
findall Method ： All match , Returns a list of
finditer Method ： All match , Return iterator
split Method ： Split string , Returns a list of
sub Method ： Replace

match Method

match Method to find the header of a string （ You can also specify the starting position ）, It's a match , As long as a match is found, it will return , Instead of finding all the matches . Its general usage is as follows ：
string String to match
pos Starting position of string , The default value is 0
endpos The end of the string , The default value is len ( String length )

group([group1, …]) Method is used to get a string that matches one or more groups , When you want to get the whole matching substring , Can be used directly group() or group(0);

start([group]) Method is used to get the starting position of the substring in the whole string （ Index of the first character of the substring ）, Parameter default The value is 0;

end([group]) Method is used to get the end position of the substring in the whole string （ Index of the last character of the substring +1）, Parameters The default value is 0;

span([group]) Method returns (start(group), end(group)).

search Method

search Method is used to find any location of a string , It's also a match , As long as a match is found, it will return , Instead of looking for all Matching results , Its general usage is as follows ：
When the match is successful , Return to one Match object , If there is no match , Then return to None.

findall Methods and finditer Method

split Method

split Method to split the string according to the matching substring and return the list , It is used in the following form ：
maxsplit Specify the maximum number of divisions , Do not specify to split all

sub Method

sub Method to replace . It is used in the following form ：

repl It can be a string or a function ：
1). Is string , Use repl To replace every matching substring of a string , And return the replaced string ,
2). If repl Is the function , This method should take only one parameter （Match object ）, And return a string to replace .

count Used to specify the maximum number of replacements , Replace all if not specified .

Greedy mode and non greedy mode :abbbc

1. Greedy mode ： On the premise that the whole expression matches successfully , As many matches as possible ( .* );
   Regular expressions using greedy quantifiers ab* , Matching results ： abbb.
   • Decided to match as much as possible b, therefore a All the back b There were .
2. Non greedy model ： On the premise that the whole expression matches successfully , As few matches as possible ( ? );
   Regular expressions using non greedy quantifiers ab*? , Matching results ： a. .*? .+?
   Even if there is * , however ? Decided to match as little as possible b, So there was no b.
3. Python The quantifier is greedy by default .