javascript regex : only english letters allowed

Another option is to use the case-insensitive flag i, then there's no need for the extra character range A-Z.

var reg = /^[a-z]+$/i;
console.log( reg.test("somethingELSE") ); //true
console.log( "somethingELSE".match(reg)[0] ); //"somethingELSE"

Here's a DEMO on how this regex works with test() and match().

let res = /^[a-zA-Z]+$/.test('sfjd');
console.log(res);

Note: If you have any punctuation marks or anything, those are all invalid too. Dashes and underscores are invalid. \w covers a-zA-Z and some other word characters. It all depends on what you need specifically.