Re: in the elemetns of programming perl , I need help with 2 lines that I don't understand

[Richard Lee]

Rob Dixon wrote:

qr// isn't often necessary, as they are usually defined, compiled and used at
the same point, like constants. However, if you have a regex that you need to
use in several places in your code then it can be useful to declare it
separately, like

  my $bracketed = qr/^\{.*\}$/;

  foreach my $element (@a) {
    print if $element =~ $bracketed;
  }

  foreach my $element (@b) {
    print if $element =~ $bracketed;
  }

It can also be useful for building complex regexes:

my $number = qr/\b[0-9]+\b/;
my $alphachar = qr/[A-Z]/i;
my $identifier = qr/\b$alphachar\w*\b/;
my $value = qr/(?:$number|$identifier)/;
my $assignment = qr/$identifier\s*=\s*$value/;


As a separate idea, the /o qualifier on a regex asks that it is compiled only
the first time it is encountered. This makes no difference unless it contains
interpolated variables, but the behavior can be seen in this program

use strict;
use warnings;

foreach my $char ('A' .. 'Z') {
  my $re = qr/$char/o;
  print "$char" if $char =~ $re;
}
print "\n";

foreach my $char ('A' .. 'Z') {
  my $re = qr/$char/;
  print "$char" if $char =~ $re;
}
print "\n";


HTH,

Rob

Thank you Rob and Chas,

I understand now.

Rob, your last code is very helpful!!

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