Comments on: Using Scalar Variables in Regular Expressions

By: William Ward

William Ward — Thu, 26 Jun 2008 00:16:36 +0000

The contents of your variable $sepchar contain a regular expression metacharacter, so that’s like saying split(/|/, $buffer) without the backslash. To add the backslash use the \Q escape code, like this:

@row = split(/\Q$sepchar\E/, $buffer);

You can think of \Q as similar to \U which converts letters to upper case; however what \Q does is insert a \ before any non-alphanumeric characters. The \E indicates the end of the quoted portion, in this case it would be optional since it’s at the end.

By: george

george — Wed, 25 Jun 2008 20:48:11 +0000

Hi, I have this portion of code:

my $sepchar = “|”; # pipe symbol
@row = split(/$sepchar/, $buffer);

However, this doesn’t work, but

@row = split(/\|/, $buffer)

DOES work. How can I mask special chars in the variable to determine the sepchar dynamically?
Thanks for any help.

By: James

James — Fri, 20 Jun 2008 16:49:34 +0000

Joe,
You can actually do this quite simply with the map function. However, I really doubt this will be any faster.

my @array = (…);
@array = map {$_ =~ s/\r//g} @array;

You’ve probably figured this out by now, but it may be useful for other people stopping by.

By: William Ward

William Ward — Thu, 28 Feb 2008 02:41:56 +0000

I don’t have any background in tcl, so I’m not spoiled the way you are :)

I’m pretty sure Perl doesn’t have a way to do what you’re asking for - only various forms of loops (foreach, grep, or map). In certain situations you could join the array elements into a string and apply your regex on that, I suppose.

You might want to ask your question on perlmonks.org.

By: Joe

Joe — Wed, 27 Feb 2008 10:34:59 +0000

I realise this is an old posting but i came here via a google search regarding scalar vars and regexp, coming from a tcl enviroment i have been looking for a way to apply a regexp substitution to an array as a whole vs looping through each item in perl. In terms of performance tcl blows it away in this context, im for certain though that if i could apply the regexp to the entire array in one iteration it would be the opposite result. The question is, how is it possible?

because it wont let you do

@array =~ s/\r//g;

versus

foreach $item @array { $item =~ s/\r//g; }

the ladder takes a substantially longer time with 100,000 lines of text than its tcl equivlant.

By: William Ward

William Ward — Sat, 16 Sep 2006 05:59:53 +0000

Yes, of course you’re right. I wonder how I missed that! Thanks. I edited the post with a line through the mistake.

By: bla

bla — Sun, 27 Aug 2006 18:28:54 +0000

this should say “Single-quoted strings are used when setting $regex to avoid needing an extra backslash.” not “Double-quoted strings are used when setting $regex to avoid needing an extra backslash.”

By: William Ward

William Ward — Mon, 30 Jan 2006 10:11:27 +0000

Excellent point. However there are a couple of minor problems with your code:

You used = instead of => in the “use constant” lines
You need to add a backslash (\) before the constant names in order to make a scalar reference that can be dereferenced.

Here’s the corrected code:

use constant NORNAL_REQ => 1;
use constant ADD_REQ => 2;
use constant FAILED_REQ => 3;

my $test = 2;
SWITCH: for($test) {
/^${\NORMAL_REQ}$/x && do { print “case 1\n”; last SWITCH;};
/^${\ADD_REQ}$/x && do { print “case 2\n”; last SWITCH;};
/^${\FAILED_REQ}$/x && do { print “case 3\n”; last SWITCH;}
}

By: Michael

Michael — Mon, 30 Jan 2006 09:01:46 +0000

There is a more interesting case: using constant in the regexp. For example, I have constants:
use constant NORNAL_REQ = 1; use constant ADD_REQ = 2; use constant FAILED_REQ = 3;
And then I’d like to use them in the one of representation of SWITCH block:
my $test = 2; SWITCH: for($test) { /^${NORMAL_REQ}$/x && do { do something for case 1 … ; last SWITCH;}; /^${ADD_REQ}$/x && do { do something for case 2 … ; last SWITCH;}; /^${FAILED_REQ}$/x && do { do something for case 3 … ; last SWITCH;} }