stripping char from an email subject line and storing it as a variable

I need for procmail to read only the first few characters (left to right) 
of an email subject line and store it as a variable. I already know
how to save the parsed data as a variable. What I need help with
is how, given the following subject line

Subject:ref215 2002-06-10 14:33:34

I can "grab" just the 'ref215' characters before the single space. 
BTW, 'ref###' if a reference number that will change (increment) 
with each incoming email. Also, I can change the blank space 
after 'ref###' with any character if that would make the recipe
argument simpler.

Thanks in advance!


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