How to perform join of lines on many rows following simple rules?

I have the following data:

YYY AAA
YYY BBB
CCCCC
YYY CCC
DDDD
EEEE

(and few hundreds rows)

Rules:
1. First line always starts with YYY.
2. Line should start with YYY.
3. All lines that do not start with YYY must be joined into previous lines.

Above sample data should become:

YYY AAA
YYY BBB CCCCC
YYY CCC DDDD EEEE

How to accomplish transformation?
Regards

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