How to use regular expressions in javascript?

This section talks about the use of regular expressions in js. Note that the regular object is a reference type.

Initialization: Use of literals and objects

Like the definition of objects, we can use literals to define regular expressions, or we can use regular objects to define regular expressions. The difference between them is There are some differences in the escaping used.

When using literals, use / / to wrap the regular expression. You can write the matching pattern or not. If not, the default is not global matching. When we use it, we usually use Literal initialization, because it is simpler, and escaping is not as complicated as strings.

// 正则表达式 /at/g 表示匹配所有at  /g表示全局搜索
var pattern1 = /at/g; // 注意不用字符串
var pattern2 = RegExp('/at', 'g');

The following need to be escaped:

Attributes of regular objects

Since the attributes of regular objects can be seen in literal variables, so These methods are not very useful, just know them:

exec() method (capture method)

The execution result of the exec method is not all matching items, but The return value of the first matched item is as shown in the following code (about the concept of capturing group, we will use it later. Here we know that only one will be matched at a time, and it will be returned at the 0th position of the array)

// exec的返回值
var text = 'mom and dad and baby';
var pattern = /mom( and dad( and baby)?)?/gi;

var matches = pattern.exec(text);
alert(matches.index); // 0 // 拥有index属性标识匹配项再字符串中的位置
alert(matches.input); // mom and dad and baby // 标识原字符串
alert(matches[0]); //mom and dad  // 标识匹配的字符串
alert(matches[1]); //and dad  //捕获组匹配的字符串
alert(matches[2]); //and baby

The exec method should pay attention to the impact of the global flag /g on the execution results. When /g is not written, there is no global matching. Each execution starts from the beginning. If /g is written, the matching is global. The next match will continue based on the previous one. This situation can be explained by looking at the example below

// 不写/g  每一次都是从头开始匹配
var text = 'cat, bat, sat, fat';
var pattern1 = /.at/;
var matches = pattern1.exec(text);
alert(matches.index); //0
alert(matches[0]); //cat
alert(pattern1.lastIndex); //0   //lastIndex方法是问题的关键，注意它是pattern1对象的属性，其实就记录了这个正则匹配到哪个位置了，这里都是0说明是一直从零开始匹配

matches = pattern1.exec(text);
alert(matches.index); //0
alert(matches[0]); //cat
alert(pattern1.lastIndex); //0

Add /g and let’s see how the results change

var text = 'cat, bat, sat, fat';
var pattern1 = /.at/g;
var matches = pattern1.exec(text);
alert(matches.index); //0
alert(matches[0]); //cat
alert(pattern1.lastIndex); //3 //lastIndex标识经过了这次匹配，下次匹配应该从什么位置开始进行

matches = pattern1.exec(text);
alert(matches.index); //5  // 标识匹配到的位置
alert(matches[0]); //bat
alert(pattern1.lastIndex); //8

test method

The test method is used to return true false after matching. If you only want to know whether there is a match and don't want to know the specific results, use this. Often used in if statements to determine whether user input is valid, etc.

var text = '000-00-0000';
var pattern = /\d{3}-\d{2}-\d{4}/;

if (pattern.test(text)) {
alert('matched');  //matched
}

Global attributes

Through the properties of RegExp, we can get some global regular information, such as what string was input just matched and what information was matched last time. This is a bit like global Static variables, let’s see what information global attributes support.

Note that global attributes can be obtained using both long and short attribute names. Let’s take a look at the use of long and short attributes

Unsupported functions

Related tutorial recommendations: JavaScript video tutorial

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