Regular Expression Mastery

Lookahead Assertions

(?=...) and (?!...) are similar to \b and \B.
They look ahead in the string to see if what follows matches ...
- If so, they succeed, but don't advance the cursor
Example: Split an email header into fields:

     Received: from ni-s.u-net.com ([193.119.182.90] helo=bactrian.ni-s.u-net.com)
             by he101war.uk.vianw.net with esmtp (Exim 3.22 #5)
             id 17H8J0-0005po-00; Sun, 09 Jun 2002 20:24:51 +0100
     Content-Disposition: inline
     Content-Transfer-Encoding: binary
     MIME-Version: 1.0
     X-Mailer: Id: //depot/mail/tkmail#119 /Perl5.008 Mail::Internet v1.46
     Subject: Re: Standard layers, documentation
     In-Reply-To: <20020609191647.GE31617@ool-18b93024.dyn.optonline.net> from
         Michael G Schwern on Sun, 09 Jun 2002 15:16:47 -0400
     Content-Type: text/plain; charset="UTF-8"
     To: schwern@pobox.com

Wrong: split /\n/
- (Consider the Received line for example)
Also wrong: split /\n\S/