Javascript正则表达式：找到第一个'use strict'

Question

I'd like to determine whether the file begins with zero or more single-line comments (the // style), followed by "use strict".

Here's my attempt:

var regex = new RegExp("(\/\/.*$)*\"use strict\";", 'm');
var codeblock = 
`
//hi
//there
"use strict";
`;

let match = regex.exec(codeblock);
console.log(match.index);

I'd expect match.index to be 0. However, it actually returns 14, meaning it doesn't recognize my comments. I suspect I didn't use the repeating pattern ()* correctly, but I'm not sure what I should be using?

Answer 1

The point is that your (\\/\\/.*\\r?\\n)* part does not match the comments since you are not consuming the linebreaks, you only assert their presence with $ . Thus, Group 0 value is undefined (try printing match[1] ) and the match is found at Position 14, ie "use strict" .

Replace $ with \\r?\\n and make sure you have no newline at the start of the test string, and you will get 0 :

 var regex = /(\\/\\/.*\\r?\\n)*\\"use strict\\";/m; var codeblock = `//hi //there "use strict";`; let match = regex.exec(codeblock); console.log(match.index); 

I also suggest using a regex literal notation /(\\/\\/.*\\r?\\n)*\\"use strict\\";/m rather than the RegExp constructor notation since your pattern is static (no variables are used to build the regex).