EDD filter purchase form submit button

gistfile1.php

<?php // DON'T COPY

function sd_purchase_button_class( $input ) {
	$input = str_replace( 'class="edd-submit', 'class="edd-submit new-class', $input );
	return $input;
}
add_filter( 'edd_checkout_button_purchase', 'sd_purchase_button_class' );