Pattern to match optional lines in multiline messages

[eik3 (Eike Herzbach)]

Here's the pattern

a:%{NOTSPACE:a}
(b:%{NOTSPACE:b})?
c:%{NOTSPACE:c}

This matches

a:1

c:42

and

a:1
b:23
c:42

But not

a:1
c:42

How can I make the pattern match the message even it the line b is completely missing?

[eik3 (Eike Herzbach)]

Got it: The pattern has to be a single line:
a:%{NOTSPACE:a}\n(b:%{NOTSPACE:b}\n)?c:%{NOTSPACE:c}