matching arbitrary text between begin and end marker

matching arbitrary text between begin and end marker

I am trying to create a new grammar for a custom DSL using xtext.
The fragment I am trying to parse looks like this:
begin string arbitrary text
more arbitrary text
end string
At the end it should be interpreted as "arbitray text\nmore arbitrary
text\n".
What is the correct way to specify something like this in xtext?
Bonus points for pointers to a good documentation. ;)