const regex = /.*\.(\w{3,})$/gmi;
// Alternative syntax using RegExp constructor
// const regex = new RegExp('.*\\.(\\w{3,})$', 'gmi')
const str = `http://www.w3schools.com/jsref/jsref_regexp_exec.asp
https://translate.google.com/#en/pt/Thank%20you%20Engr!%20That%20is%20awesome.php
www.facebook.com/index.asp`;
const subst = `$1`;
// The substituted value will be contained in the result variable
const result = str.replace(regex, subst);
console.log('Substitution result: ', result);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for JavaScript, please visit: https://developer.mozilla.org/en/docs/Web/JavaScript/Guide/Regular_Expressions