Related understanding of regular expressions

This article will explain in detail the relevant knowledge of regular expressions.

What are \d, \w, \s, [a-zA-Z0-9], \b,.,*, ,?,x{3},^,$ respectively?

\d: Metacharacter, matches a number, equivalent to [0-9] (matches one from 0 to 9);
\w: Metacharacter, matches letters or numbers or underscores or Chinese characters, equivalent to [0-9a-zA-Z_];
\s: metacharacter, matches any whitespace character;
[a-zA-Z0-9]: [] specifies a range, Matches one of them. The example matches one of a-z/A-Z/0-9, which is equivalent to \w (except Chinese characters);
\b: metacharacter, matches the beginning or end of a word (word boundary):

var a= "hello helloworld";var reg = /\bhello\b/;
a.match(reg);//The result is "hello";

.: metacharacter, matches all characters except newlines;
*: qualifier, repeated 0 or more times;
: qualifier, repeated 1 or more times, at least 1 time;
? : Qualifier, repeated 0 or 1 times;
x{3}: Qualifier, x appears 3 times ({n} repeated n times; {n,m} repeated n-m times (including n,m); { n,} is repeated at least n times; {,m} is repeated at most m times);
: means negation in [] ([abc] matches any character in abc, [abc] matches any character except abc) ;Other times, it can match the beginning of the string;
$: Match the end of the string; (^hello&: Match the string starting with hello and ending with hello)

Write a function trim(str), Remove the blank characters on both sides of the string

function trim(str) {    return str.replace(/^\s+|\s+$/g,'') //匹配开头或结尾的空白字符，替换成'';
 }

Write a function isEmail(str) to determine whether the user input is an email address

function isEmail(str) {    var reg = /^[a-zA-Z\d_]+\@[a-zA-Z\d]+\.[a-zA-Z\d]+$/g;    return reg.test(str);
 }

Write a function isPhoneNum(str) to determine whether the user input is an email address Mobile phone number

function isPhoneNum(str) {    var reg = /^1[3578]\d{9}$/g;    return reg.test(str);
}

Write a function isValidUsername(str) to determine whether the user input is a legal username (length 6-20 characters, can only include letters, numbers, and underscores)

function isValidUsername(str) {    var reg = /^([a-zA-Z\d_]){6,20}$/g;    return reg.test(str);
}

Write a function isValidPassword(str) to determine whether the user enters a legal password (6-20 characters in length, including only uppercase letters, lowercase letters, numbers, and underscores, and at least two types)

function isValidPassword(str) {    if (/^[a-zA-Z0-9_]{6,20}$/g.test(str)) {        if (/^[a-z]{6,20}$/g.test(str) || /^[A-Z]{6,20}$/g.test(str) || /^[0-9]{6,20}$/g.test(str) || /^[_]{6,20}$/g.test(str)) {            return false;
        }else {            return true;
        }
    }else {        return false;
    }
}

Write a regular expression to get all the colors in the following string

var re = /*正则...*/var subj = "color: #121212; background-color: #AA00ef; width: 12px; bad-colors: f#fddee "console.log( subj.match(re) )  // ['#121212', '#AA00ef']
var re = /#[a-f\d]{6}/ig;var subj = "color: #121212; background-color: #AA00ef; width: 12px; bad-colors: f#fddee";console.log( subj.match(re) )

What does the following code output? Why? Rewrite the code so that it outputs [""hunger"", ""world""]

var str = 'hello  "hunger" , hello "world"';var pat =  /".*"/g;
str.match(pat);
//输出[""hunger" , hello "world""];

//Regular expressions are in greedy mode by default and will match as many matches as possible if the conditions are met;
//Rewrite the code

var str = 'hello  "hunger" , hello "world"';
var pat =  /".*?"/g;  //添加?改成非贪婪模式，尽可能少的匹配；
str.match(pat); //[""hunger"", ""world""]

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