var regex = new RegExp('[abc]');
console.log(regex.test('a')); // true, 1 match

var regex = new RegExp('[abc]');
console.log(regex.test('ac')); // true , 2 matches

var regex = new RegExp('[abc]');
console.log(regex.test('Hey Jude')); // false. , no match

var regex = new RegExp('[abc]');
console.log(regex.test('abc de ca')); // true , 5 matches

var regex = new RegExp('[a-e]');  // a-e means abcde
var regex = new RegExp('[1-4]');  // 1-4 means 1234
var regex = new RegExp('[0-39]');  // 0-39 means 01239
var regex = new RegExp('[0-9]');  // 0-9 means 0123456789