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From: jes@mbio.med.upenn.edu (Joe Smith)
Newsgroups: comp.lang.perl
Subject: Re: global pattern matching question
Message-ID: <JES.91May5190550@mbio.med.upenn.edu>
Date: 5 May 91 23:05:50 GMT
References: <JES.91May3121528@mbio.med.upenn.edu>
	<1991May4.010307.11792@jpl-devvax.jpl.nasa.gov>
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In-reply-to: lwall@jpl-devvax.jpl.nasa.gov's message of 4 May 91 01:03:07 GMT


In article <1991May4.010307.11792@jpl-devvax.jpl.nasa.gov>
lwall@jpl-devvax.jpl.nasa.gov (Larry Wall) writes:

> In 4.003, $` is unfortunately broken within s/// because of a fix to
> something else.  That'll be fixed in patch 4.  As a workaround you
> could use length($seq) - length($')  - length($1).  Yeah, blech...

Oops, I just saw the other bug report.

The book says that $` and $' aren't generated unless they're
mentioned.  In an expression like length $`, does perl actually
generate a copy of this piece of the original string just to determine
the match position, or is the length just calculated from the match
position?

> An option to s/// not to do replacement would be interesting, though

And to think I've always wondered how such wonderfully
convoluted language design happens...

> there might be a better way--maybe an initial offset for m//, or
> allowing patterns in index().

This seems more direct, but the ability to bind an action to the match
is also very important.  Perhaps access to a primitive pattern
matching function (the way splice is related to push, pop, etc.) would
be reasonable:

   $options = "oi";

   for ($pos = 0;
	   $matched = regexp( $pattern, $seq, $pos, $options );
	   $pos += $matched ) {
		   push( @positions, $pos );
		   push( @sites, subst($seq, $pos, $matched ));
   }

I think you could replicate the match and substitute capabilities with
that (subst($seq, $pos, $matched ) = "your replacement here..."), with
arbitrary actions on a match and without a lot of extra string copies.
It'll never fit on one line, but I could live with that.

<Joe
--
 Joe Smith
 University of Pennsylvania                    jes@mbio.med.upenn.edu
 Dept. of Biochemistry and Biophysics          (215) 898-8348
 Philadelphia, PA 19104-6059