Introduction and underlying analysis of regular expressions

Regular expression Introduction

Also known as regular expression （ English ：Regular Expression, In code it is often abbreviated as regex、regexp or RE）, Computer science A concept of . Regular expressions are often used for retrieval 、 Replace those that match a pattern ( The rules ) The text of .

Many programming languages support string manipulation with regular expressions . for example , stay Perl A powerful regular expression engine is built in . The concept of regular expression was originally developed by Unix Tool software in （ for example sed and grep） Popular . Regular expressions are usually abbreviated to “regex”, singular Yes regexp、regex, The plural Yes regexps、regexes、regexen.

Regular expressions are easy to use

Suppose you use a crawler to crawl this content on the website , How to quickly get the English letters or numbers in this paragraph ？ Regular expressions are used at this time

Python from Netherlands mathematics and Computer science Research Society Guido · Van rosum On 1990 It was designed in the early 's , As a course called ABC Language substitute . [1] Python Provides an efficient advanced data structure , It can also simply and effectively object-oriented Programming .Python Syntax and dynamic types , as well as Explanatory language The essence of , Make it written on most platforms Script And a programming language for rapid application development , [2] With the continuous update of the version and the addition of new language features , Gradually used for independent 、 A large project Development of . [3] Python Interpreter extensible , have access to C Language or C++（ Or something else can be done through C Calling language ） Expand new features and data type . [4] Python It can also be used as an extensible programming language in customizable software .Python Rich library of standards , It provides a platform for each main system platform Source code or Machine code . [4] 2021 year 10 month , Compiler for language popularity index Tiobe take Python Crowned the most popular programming language ,20 Put it in... For the first time in years Java、C and JavaScript above .

package regularexpression;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class RegularExpression {
    
    public static void main(String[] args) {
    

        // Suppose you use a crawler to crawl to this content 
        String context = "Python By Guido of the Dutch Society for mathematical and computer science research · Van rosum   On 1990  It was designed in the early 's ," +
                " As a course called ABC A substitute for language . [1] Python Provides efficient advanced data structure , It can also simply and effectively ground " +
                " Programming objects .Python Syntax and dynamic types , And the nature of interpretative language , Make it easy to write scripts and fast on most platforms " +
                " Programming language for developing applications , [2]  With the continuous update of the version and the addition of new language features , Gradually used for independent 、 A large project " +
                " Development of . [3] "+"Python The interpreter is easy to extend , have access to C Language or C++（ Or something else can be done through C Calling language ）" +
                " Expand new functions and data classes " +
                " type . [4] Python  It can also be used as an extensible programming language in customizable software .Python Rich library of standards , Provides a " +
                " Source code or machine code of three main system platforms . [4] " +
                "2021 year 10 month , Compiler for language popularity index Tiobe take Python Crowned the most popular programming language ,20 For the first time in years " +
                " On Java、C and JavaScript above . [16] ";
        // Extract all English words in the article 
        // So let's create one Pattern object 
        Pattern pattern = Pattern.compile("[a-zA-Z]+");
        // Create a match object 
        // Namely matcher  The matcher follows pattern Model claims , To context Match in text 
        // Find and return true  Return if not found false
        Matcher matcher = pattern.matcher(context);
        // Begin to match 
        while (matcher.find()){
    
            // The matching content will be in matcher.group(0) in 
            System.out.println(" find  " + matcher.group(0));
        }


    }
}

The running result is

When we need to get the numbers in the text , Just put this statement

Pattern pattern = Pattern.compile("[a-zA-Z]+"); 

Replace with

Pattern pattern = Pattern.compile("[0-9]+");

The running result is

If you want to capture the year related, you will

Pattern pattern = Pattern.compile("[0-9]+");

Replace with

\ \d Represents any number

Pattern pattern = Pattern.compile(""\\d\\d\\d\\d"");

The running result is

analysis

martch.find() The task accomplished

1. According to rules , Locate the substring that meets the rule
2. After finding , Record the index of Zizi very beginning to matcher Object properties int[] groups; groups[0] = 31 , Put the word string of lodging Indexes +1 The value of is recorded to groups[1] = 35

Perform breakpoint debugging here , It turns out that there is one groups

When we took a step forward, we found groups Array groups[0] = 31groups[1] = 35

Is precisely 1990 Index position in text .

3. It will also record oldLast Value , Ending with a substring Indexes +1 The value is 35 , That is, the next execution find When , From 35 This position starts to match .
   

march.group(0) analysis

We go through group Source code

public String group(int group) {
    
    if (this.first < 0) {
    
        throw new IllegalStateException("No match found");
    } else if (group >= 0 && group <= this.groupCount()) {
    
        return this.groups[group * 2] != -1 && this.groups[group * 2 + 1] != -1 ? this.getSubSequence(this.groups[group * 2], this.groups[group * 2 + 1]).toString() : null;
    } else {
    
        throw new IndexOutOfBoundsException("No group " + group);
    }
}

1. according to groups[0] = 31 and groups[1] = 35 The location of the record , from content Start to intercept the string and put it back . That is to say, what is returned is [31,35) The position of the index in the interval .
2. If the execution continues find Method Still follow the above rules .

When will

Pattern pattern = Pattern.compile(""\\d\\d\\d\\d"");

Change to grouping , The first bracket is the first group , The second bracket is the second group .

Pattern pattern = Pattern.compile("(\\d\\d)(\\d\\d)");

1. According to rules , Locate the substring that meets the rule

2. After finding , Record the index of Zizi very beginning to matcher Object properties int[] groups; groups[0] = 31 , Put the Indexes +1 The value of is recorded to groups[1] = 35 .

3. Record the first group () Matching string groups[2] = 31groups[3] = 33

   Because the matching string is 1980 therefore 19 Is the location of the first group , Again because 1 The index is 31,9 The index is 32 therefore groups[2] = 31 groups[3] = 33

4. Record the second group () Matching string groups[3] = 33groups[4] = 35

5. If there are more groups, and so on

Pictured

take while Change the code in to

while (matcher.find()){
    
    // The matching content will be in matcher.group(0) in 
    System.out.println(" find  " + matcher.group(0));
    System.out.println(" The first group () The match is  " + matcher.group(1));
    System.out.println(" The second group () The match is  " + matcher.group(2));

}

The running result is

Summary ： If the regular expression has () That is, grouping , The extracted string rule is group(0) Represents the matched string ,group(1) Represents the first set of characters in the matched substring ,group(2) Represents the second set of characters in the matched substring . Be careful ！！ The number of groups cannot exceed the limit .