Global matching pattern analysis of regular expressions in Javascript_javascript skills

Copy code The code is as follows:

var str = "123#abc";
var re = /abc/ig;
console.log(re.test(str)); //Output true
console.log(re.test(str)); //Output false
console.log( re.test(str)); //Output true
console.log(re.test(str)); //Output false

If used when creating a regular expression object When the "g" identifier or its global attribute value is set to true, the newly created regular expression object will use the pattern to globally match the string to be matched. In global matching mode, multiple matches can be performed on the specified string to be found. Each match uses the value of the lastIndex attribute of the current regular object as the starting position to start searching in the target string. The initial value of the lastIndex attribute is 0. After a matching item is found, the value of lastIndex is reset to the position index of the next character of the matching content in the string, which is used to identify the position where to start searching the next time a match is performed. If no matching item is found the value of lastIndex will be set to 0. When the global matching flag of the regular object is not set, the value of the lastIndex attribute is always 0, and each time a match is performed, only the first matching item in the string is found. You can use the following code to view the value of the corresponding lastIndex attribute that matches the execution.

Copy code The code is as follows:

var str = "123#abc";
var re = /abc/ig;
console.log(re.test(str)); //Output ture
console.log(re.lastIndex); //Output 7
console.log( re.test(str)); //Output false
console.log(re.lastIndex); //Output 0
console.log(re.test(str)); //Output true
console.log(re.lastIndex); //Output 7
console.log(re.test(str)); //Output false
console.log(re.lastIndex); //Output 0

About the RegExp.prototype.exec(str) method and the String.prototype.math(rgExp) method

The test method of the regular object returns a value of true or false, and only needs to detect the target This method is useful when checking whether a string matches a specified pattern, but does not need to obtain the matching content. When you need to obtain the matching result, you need to use the exec(str) method of the RegExp type or the match(rgExp) method of the String type.

The RegExp.prototype.exec(str) method returns NULL or an array. The 0th element of the array stores the matching content found in the string str. 1 to n elements are returned. is the content of the submatch specified using brackets "()" in the pattern.

The String.prototype.math(rgExp) method behaves similarly to RegExp.prototype.exec(str) when no global flag is used. When the global match flag is set, the array item elements 0 to n returned by the String.prototype.math(rgExp) method contain all matched items excluding submatches. At this time, you can use RegExp.$1..$9 to obtain 9 submatches.