From: Ross Bamford <rossrt@...>
Date: 2006-04-12T21:21:03+09:00
Subject: Re: Simple substitutions

Hi,

On Wed, 2006-04-12 at 20:58 +0900, Peter Bailey wrote:

> I have a need to parse postscript print files on a daily basis. Some of 
> them are over 10 mB. One thing I need to find out is the number of pages 
> in the files. There's a simple one-liner at the top of each .ps file 
> that states the number of pages. It reads:
>                              %%Pages: xx
> It's a line of its own.
> 
> I simply want to find that line in my source file and then write a 
> corresponding line in my output file:
>                              Number of pages: xx
> 
> I tried the following in IRB, but I get the syntax error shown below.
> 
>      File.open("psout.txt", "w") do |output|
>        File.foreach("test1.ps") do |line|
>          line.sub!(/\%\%Pages: ([0-9]{1,5})/"Pages: \1"/)
>          output << line
>        end
>      end
> 
>     SyntaxError: compile error
>     (irb):26: parse error, unexpected tSTRING_BEG, expecting ')'
>     line.sub!(/\%\%Pages: ([0-9]{1,5})/"Pages: \1"/)
>                                     ^
>     (irb):26: parse error, unexpected ')'
>            from (irb):29
> 
> Is there a significance to the ^ under the "P" in my syntax error? 
> What's it telling me? My sub requires parentheses. Is there a conflict 
> with my grouping parentheses inside my regular expression?
> 

ruby is just complaining about a Perl-style s/// regexp. :) The correct
syntax here would be something like:

	line.sub!(/\%\%Pages: ([0-9]{1,5})/) { "Pages: #$1" }

(You can sometimes use the \1 escapes in the replacement but it's always
come out hit-and-miss for me, so I stick to the block form).

> Another request for help--my translation above would basically move the 
> entire contents of my source file to the target file, with the exception 
> of the one line I want changed. That's overkill for me. I just want a 
> few lines of information in my target file. Would you suggest I just do 
> a simple search, using matching (=~), and then write about that find in 
> my target file, or, would it be better to do mass deletions of 
> everything I don't want from my source to my target files? In other 
> words, should I say "in this file, find this, then say this . . ." or, 
> "convert this entire file, but just change this?"

I would probably go with extracting just what I needed from the file,
though if you can safely exclude portions of it before running matches
(for example a large data section) that may improve performance.

Also, I don't know if there are any Ruby postscript libraries, but it
might be worth a search if you haven't already...

Hope that helps,
-- 
Ross Bamford - rosco@roscopeco.REMOVE.co.uk