Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!convex!usenet
From: tchrist@convex.COM (Tom Christiansen)
Newsgroups: comp.editors
Subject: Re: Multiple line regexps
Keywords: regexp, multiple lines
Message-ID: <1991Jun03.000600.21967@convex.com>
Date: 3 Jun 91 00:06:00 GMT
References: <1991Jun2.231351.10229@trl.oz.au>
Sender: usenet@convex.com (news access account)
Reply-To: tchrist@convex.COM (Tom Christiansen)
Distribution: comp
Organization: CONVEX Software Development, Richardson, TX
Lines: 30
Nntp-Posting-Host: pixel.convex.com

From the keyboard of soh@andromeda.trl.OZ.AU (kam hung soh):
:I would like to write a regular expression which can look for patterns
:longer than one line.  For example, I want to find the first line of
:each paragraph.  If I try this regexp in grep or awk, /^$^.+$/, nothing
:happens.  Admittedly, I could replace newlines with a unique character
:say '~', before I process my file, but I wondered if regexps can be used
:across a newline boundary.

Not in most text processing languages, but I'll offer two alternatives.

Rob Pike once explained to me that his screen editor, sam, could handle
such things because it doesn't have hard-wired in what a line is.  Sadly,
sam is not available for free (although it's probably cheap from the 
AT&T toolbox) so I've not used it.  Perhaps someone who has might comment.

Another possibility is to use perl, which isn't really an interactive
editor, but is certainly a superset of sed, awk, and sh, at the very
least.  Perl has no problems with regexps spanning multiple lines.
While the default records processed are a line at a time, you can switch
this to paragraph mode (records delimited by newline pairs) or even
whole-file mode, in which you can slurp the entire file into the pattern
space.  There's no problem saying something like s/\n\nX\n//g then.  In
fact, there's an internal variable you can set to change the definitions
of ^ and $ to mean not just at the beginning or end of a string, but
rather anywhere after and before a newline as well, which is often handy.

--tom
--
Tom Christiansen		tchrist@convex.com	convex!tchrist
	    "Perl is to sed as C is to assembly language."  -me