ipi-jquery/exemples/validation-de-formulaires-bootstrap/script.js

$('#exampleInputUsername1')
.change(function(e) {
    var inputUsername = $(this);
    var username = inputUsername.val();
    var re = /^[A-Za-z][A-Za-z0-9_]+$/;
    var isUsernameValid = username.match(re);
    if(! isUsernameValid) {
        inputUsername
        .addClass('is-invalid')
        .removeClass('is-valid');
    }
    else {
        inputUsername
        .addClass('is-valid')
        .removeClass('is-invalid');
    }
    
    
})