Does anyone here have any idea how to construct a regular expression to match an URL broken in this way, with an linebreak at an arbitrary position?

Yes, I do. But I'm not going to. Because it will be one beast of a regular expression. For comparison, let me give you a regex for just matching HTTP URLs (that's just forgetting all the other possible schemes). Just image if you want to adapt that to include newlines...

(?:(?:http)://(?:(?:(?:(?:(?:(?:[a-zA-Z0-9][-a-zA-Z0-9]*)?[a-zA-Z0-9])
[.])*(?:[a-zA-Z][-a-zA-Z0-9]*[a-zA-Z0-9]|[a-zA-Z])[.]?)|(?:[0-9]+[.][0
-9]+[.][0-9]+[.][0-9]+)))(?::(?:(?:[0-9]*)))?(?:/(?:(?:(?:(?:(?:(?:[a-
zA-Z0-9\-_.!~*'():@&=+$,]+|(?:%[a-fA-F0-9][a-fA-F0-9]))*)(?:;(?:(?:[a-
zA-Z0-9\-_.!~*'():@&=+$,]+|(?:%[a-fA-F0-9][a-fA-F0-9]))*))*)(?:/(?:(?:
(?:[a-zA-Z0-9\-_.!~*'():@&=+$,]+|(?:%[a-fA-F0-9][a-fA-F0-9]))*)(?:;(?:
(?:[a-zA-Z0-9\-_.!~*'():@&=+$,]+|(?:%[a-fA-F0-9][a-fA-F0-9]))*))*))*))
(?:[?](?:(?:(?:[;/?:@&=+$,a-zA-Z0-9\-_.!~*'()]+|(?:%[a-fA-F0-9][a-fA-F
0-9]))*)))?))?)
[download]

Newlines added for readability only. They are not part of the regex.

Abigail