How do I match any character across multiple lines in a regular expression?

It depends on the language, but there should be a modifier that you can add to the regex pattern. In PHP it is:

/(.*)<FooBar>/s

The s at the end causes the dot to match all characters including newlines.

Try this:

((.|\n)*)<FooBar>

It basically says "any character or a newline" repeated zero or more times.