WooCommerce gettext string replacement example.

gistfile1.txt

function xx_woo_text_strings( $translated_text, $text, $domain ) {

    if ('woocommerce' == $domain){

        $strings = array('Coupons' => 'Vouchers', 'Coupon Code' => 'Voucher Code');

        foreach ($strings as $original => $updated) {
            $string_found = stristr($translated_text, $original);
            if( $string_found !== FALSE ) {
                $translated_text = str_ireplace($original, $updated, $translated_text);
                break;
            }
        }
    }

    return $translated_text;
}

add_filter( 'gettext', 'xx_woo_text_strings', 20, 3 );