Message
ext-link
xlink:href attribute "@xlink:href" should contain a valid URL including scheme (http://, https://, ...)
Rule XPath:
This rule ASSERTS the following, raising an alert if FALSE:
not(@xlink:href) or matches(@xlink:href, '^[a-z][a-z0-9+\.\-]+:.*') or (@ext-link-type
= 'doi')