JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="TextBox1" name="TexBox1" value="" />
<span id='error' name='error'></span>

JavaScript

function monitor() {
    if ($('#TextBox1').val().length > 0) {
        var reg = /^([a-zA-Z]+)$/;
        if (!reg.test($('#TextBox1').val())) {
            $('#error').html("Only letters are allowed!");
        } else {
            $('#error').html("");
        }
    } else {
        $('#error').html("");
    }
}

var timer = '';

$('#TextBox1').on('focus', function () {
    timer = setInterval(monitor, 100);
}).on('blur', function () {
    clearInterval(timer);
});