var metaRegex = new RegExp(/<link.*?href=['"](.*?)['"].*?rel=['"]canonical['"].*?>/i);
// return correctly: https://support.google.com/recaptcha/?hl=en
// var metaRegex = new RegExp(/<link(?=.*rel=['"]canonical['"])(?=.*href=['"](.*?)['"]).*?>/i);
// return incorrectly: https://www.google.com/accounts/TOS
var metaTag = metaRegex.exec(body);
console.log(metaTag[1]);

<link href="https://support.google.com/recaptcha/?hl=en" rel="canonical">

<link rel="canonical" href="https://support.google.com/recaptcha/?hl=en">

var body = '<link rel="stylesheet" href="my.css"/> <link href="https://support.google.com/recaptcha/?hl=en" rel="canonical"/> <a href="https://www.google.com/accounts/TOS"/>'
var linkRegexp = /(<link[^>]*rel=['"]canonical['"][^>]*>)/;
var hrefRegexp = /href=['"](.*?)['"]/;

var linkBody = linkRegexp.exec(body)[1];
console.log(hrefRegexp.exec(linkBody)[1]);