regex = re.compile(r'>s*</')


replaced = re.sub(regex, ">[!] JSON value does not exist, Check your Json!</", temp)

<file>
            <fileType>Mezza</fileType>
            <fileName></fileName>
            <segments>0000</segments>
        </file>

<file>
            <fileType>Mezza</fileType>
            <fileName>[!] value does not exist!</fileName>
            <segments>0000</segments>
        </file>

</fileName>[!] value does not exist!</file>