See: http://fransbouma.github.io/DocNet/markdownextensions.htm

Specifying

@tab This is a header
content
@end

will make the tab have the header 'This' and the rest of the header 'is a header' is added to the tab content.

Caused by a faulty regexp which filters in \s after the first word, which isn't sufficient. It explicitly has to match a \n at the end of the line.