HTML simple not blank pattern

HTML has the required attribute to accomplish this. If you set any input to be required, modern browsers won't let you submit the form if those fields are empty.

<input type="text" name="username" required="required" pattern="[A-Za-z0-9]{1,20}">

To prevent errors from showing on load, you can not use the HTML5 required attribute. You can use JavaScript. For example:

if ( $('#form-password').val() === "" ) 
{
    e.preventDefault();
}

Using HTML Patterns to match at least one:

<input type="text" name="username" pattern=".{1,}">