Regex - Get value from url

Question

Need explanation on Regular Expression. I have below codes,

function getURLParameter(url, name) {
    return (RegExp(name + '=' + '(.+?)(&|$)').exec(url)||[,null])[1];
}

var id = getURLParameter(url, 'id');
console.log(id); //1

// let say the example of url is 
// index.php?id=1&user=xxx

I understand the exec() function on js is to find the match.

What does the RegExp() check to get the value of 1?

I have test it on regExr.com and it get the match of id=1&.

Update Question

Sorry for the confusing question. What I need to know is on the return statement.

What is the use of [1] on

return (RegExp(name + '=' + '(.+?)(&|$)')
.exec(url)||[,null])[1];