RE: Pattern Match
From: Tom Kinzer (tomkinzer_at_earthlink.net)
Date: 12/09/03
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To: <beginners@perl.org> Date: Tue, 9 Dec 2003 12:37:59 -0800
Rob, can you explain the details of that replace? That's pretty slick. I
see you're adding the hex value to get to the appropriate ASCII value, but
didn't know you could do some of that gyration inside a regex.
Thanks.
-Tom Kinzer
-----Original Message-----
From: Rob Dixon [mailto:rob@dixon.port995.com]
Sent: Tuesday, December 09, 2003 11:58 AM
To: beginners@perl.org
Subject: Re: Pattern Match
Eric Sand wrote:
>
> I am very new to Perl, but I sense a great adventure ahead after just
> programming with Cobol, Pascal, and C over the last umpteen years. I have
> written a perl script where I am trying to detect a non-printing
> character(Ctrl@ - Ctrl_) and then substitute a printing ASCII sequence
such
> as "^@" in its place, but it does not seem to work as I would like. Any
> advice would be greatly appreciated.
>
> Thank You....Eric Sand
>
>
Your obvious guess is to write Perl as if it were C. That's slightly better
than treating it as a scripting language, but there are many joys left to be
found!
> $in_ctr=0;
> $out_ctr=0;
>
> while ($line = <STDIN>)
> {
> chomp($line);
> $in_ctr ++;
> if ($line = s/\c@,\cA,\cB,\cC,\cD,\cE,\cF,\cG,\cH,\cI,\cJ,\cK,
> \cL,\cM,\cN,\cO,\cP,\cQ,\cR,\cS,\cT,\cU,\cV,\cW,
> \cX,\cY,\cZ,\c[,\c\,\c],\c^,\c_
> /^@,^A,^B,^C,^D,^E,^F,^G,^H,^I,^J,^K,
> ^L,^N,^N,^O,^P,^Q,^R,^S,^T,^U,^V,^W,
> ^X,^Y,^Z,^[,^\,^],^^,^_/)
> {
> $out_ctr ++;
> printf("Non-printing chars detected in: %s\n",$line);
> }
> }
> printf("Total records read =
%d\n",$in_ctr);
> printf("Total records written with non-printing characters =
%d\n",$out_ctr);
I would write this as below. The first things is to *always*
use strict;
use warnings;
after which you have to declare all of your variables with 'my'.
The second is to get used to using the default $_ variable which
is set to the value for the current 'while(<>)' or 'for' loop
iteration, and is a default parameter for most built-in functions.
Finally, in your particular case you're using the s/// (substitute)
operator wrongly. The first part, s/here//, is a regular expression,
not a list of characters. You'll need to read up on these at
perldoc perlre
The second part, s//here/, is a string expression which can use
'captured' sequences (anything in brackets) from the first part
and, with the addition of the s///e (executable) qualifier can
also be an executable statement. Here I've used it to add 0x20
to the ASCII value of the control character grabbed by the regex.
A lot of this won't make sense until you learn some more, but I
hope you'll agree that this code is cuter than your original?
HTH,
Rob
use strict;
use warnings;
my $in_ctr = 0;
my $out_ctr = 0;
while (<>) {
chomp;
$in_ctr++;
if (s/([\x00-\1F])/'^'.chr(ord($1) + 0x40)/eg) {
$out_ctr++;
printf "Non-printing chars detected in: %s\n", $_;
}
}
printf "Total records read = %d\n", $in_ctr;
printf "Total records written with non-printing characters = %d\n",
$out_ctr;
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