Question

How can I use jQuery to validate (on keyup) that a text input only contains latin letters (only_english = 'abcdAbDKDk',), without using a plugin?

Answer 1

Try something like this:

$(function() {
    $(".latin-only").keypress(function(event) {
        if(!((event.which >= 65 && event.which  <= 90) ||
           (event.which >= 97 && event.which <= 122))) {
            event.preventDefault();
        }
    }); 
});

If you want to allow spaces, add that to the statement above (|| event.which == 32). You will need extra logic somewhere (perhaps in a change event, or keyup) to remove illegal chars from the string when users paste a value into the input.

JSFiddle example.