Rough and tumble quick and dirty example of how to add some custom JavaScript validation to an EE4 form field.

validation.php

// add script to registration form
add_action( 'AHEE__attendee_information__reg_step_start', 'eea_custom_validation' );

function eea_custom_validation() {
    ?>
    <script>
    jQuery(document).ready(function($) {
      $('.ee-reg-qstn-78').keyup(function() { // use your unique input class 
        $('span.error-keyup-78').remove();
        var inputVal = $(this).val();
        var characterReg = /^\s*[a-zA-Z0-9,\s]+\s*$/; // your custom regex, this checks for special characters
        if(!characterReg.test(inputVal)) {
            $(this).after('<span class="error ee-required-text error-keyup-78">Only a-z and 0-9 please.</span>');
        }
      });
      // handle the submit button
      $('#spco-go-to-step-finalize_registration-submit').click(function() {
        if($('span.error').length > 0){
          alert('Please correct the errors in the form.');
          return false;
        }
      });
    });
    </script>
    <?php
}