sed - match newlines on input

[Joseph Boykin]

In article <570 at hao.UCAR.EDU>, bill at hao.UCAR.EDU (Bill Roberts) writes:
> I'm trying to match a pattern over multiple lines.
> For instance, on the input:
> 
> one
> two
> three
> 
> with the sed script:
> 
> s/one\ntwo\nthree/one, two, three/g
> one would expect to get the following output
> one, two, three
> 
> OK, so I don't understand the manual (what else is new).  How can I get what I
> need?  Also, what about the "Multiple Input-line Functions".  Might that be the
> way to go?  An example would really help.  Thanks in advance.
> 
> 							Bill Roberts
> 							NCAR/HAO
> 							Boulder,CO


The documentation on this point within the SED documentation is
definately confusing, when we did the documentation for PC/SED,
we tried to make it better, but I don't think we did!

Okay, here goes:
SED's regular expression handler was modified to handle embedded newlines.
Compare this to VI (UNIX or PC/VI) which CANNOT match a pattern which crosses
a line boundary.  When searching for a regular expression, you search
a single null terminated string.  That string can have any character
in it, including a new-line (although for the most part the new-line isn't
stored).  Hence, your script will not work since SED is not seeing
the \n in your script as "when you get to the end of this string, start
comparing with the next string (pronounced 'line').  When the documentation
talks about looking for embedded new lines, what it is talking about
is that SED permits the user to 'join' two lines together.  What this
really means is that the NULL in the first line is replaced by a \n and
the second line is concatenated onto the end of the first.
The regular expression can now test for an embedded new line since all you have
is one string which just so happens to have a \n in the middle.

To be honest, I don't feel like mucking with SED long enough
to give you a script to do what you want (someone else probably will!)
but I think the basic idea is to go through the file, and for each
line join the next two lines together with the 'N' command, then you
can test to see if that new 'line' is the concatenation of the
three you are interested in, if so, do your substitution.

Okay, I just reread this message and I know it isn't very clear.
On the other hand, neither is SED is not the easiest program to
understand either!  If you're still stuck, give me a call.

-- 

Joe Boykin
Custom Software Systems
...{necntc, frog}!custom!boykin