txt <- "Message-ID: <24216240.1075855687451.JavaMail.evans@thyme>
Date: Wed, 18 Oct 2000 03:00:00 -0700 (PDT)
From: phillip.allen@enron.com
To: leah.arsdall@enron.com
Subject: Re: test
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
X-From: Phillip K Allen
X-To: Leah Van Arsdall
X-cc: 
X-bcc: 
X-Folder: \\Phillip_Allen_Dec2000\\Notes Folders\\'sent mail
X-Origin: Allen-P
X-FileName: pallen.nsf
 
test successful.  way to go!!!"
 
result <- regmatches(txt, regexec("X-FileName:[^\n]+\n\n(.+)$", txt))
result[[1]][2]

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