while using nested perl grep output differs. why? - regex

Question

kindly explain, why this issue comes
my data file
DATA----1
DATA----2
DATA----3
DATA----4
DATA----5
DATA----6
DATA----7
SAMPLE----1
SAMPLE----12
SAMPLE----13
SAMPLE----2
SAMPLE----3
SAMPLE----4
SAMPLE----5
OTHER----1
OTHER----2
OTHER----3
where I need entire line which start with DATA and SAMPLE to an array and an another array should have content which start with SAMPLE end with two digit number
I have got output with following script
use strict;
use warnings;
open(FH, "di.txt");
my #file = <FH>;
close(FH);
my #arr2 = grep { $_ =~ m/^SAMPLE.+\d\d$/g } #file; ## this array prints
my #arr1 = grep { $_ =~ m/^DATA|^SAMPLE/g } #file;
print #arr1,"\n\t~~~~~~~~~~~\n\n",#arr2;
First writen as
use strict;
use warnings;
open(FH, "di.txt");
my #file = <FH>;
close(FH);
my #arr1 = grep { $_ =~ m/^DATA|^SAMPLE/g } #file;
my #arr2 = grep { $_ =~ m/^SAMPLE.+\d\d$/g } #file; ## this doesn't print
print #arr1,"\n\t~~~~~~~~~~~\n\n",#arr2;
while run this one, prints only #arr1
what would be the reason #arr2 don't print

Answer 1

The problem is because of the behaviour of the global match /g option in scalar context
Every scalar variable has a marker that remembers where the most recent global match left off, and hence where the next one should start searching. It enables the use of the \G anchor in regex patterns, as well as while loops like this
my $s = 'aaabacad';
while ( $s =~ /a(.)/g ) {
print "$1 ";
}
which prints
a b c d
In truth you're not interested in a global match in this case, you just want to discover whether OR NOT the pattern can be found in the string. The grep operator applies scalar context to its first parameter, so in using the /g option in this statement
my #arr1 = grep { $_ =~ m/^DATA|^SAMPLE/g } #file;
you have left every element of the #file with the marker set to right after DATA or SAMPLE. That means the next match on the same element m/^SAMPLE.+\d\d$/g will start looking from there and clearly can't even find the ^ anchor to the match fails
The pos function gives you access to the marker, and you can fix your original code by resetting it to the start of the string after the first grep call. If you write this instead
my #arr1 = grep { $_ =~ m/^DATA|^SAMPLE/g } #file;
pos($_) = 0 for #file;
my #arr2 = grep { $_ =~ m/^SAMPLE.+\d\d$/g } #file; ## this doesn't print
then the output will be what you expected
The correct fix, however, is to write what you mean anyway, which means you should remove the /g option from the pattern matches. This code also works fine, and it's also more concise, more readable, and far less fragile
my #arr1 = grep /^DATA|^SAMPLE/, #file;
my #arr2 = grep /^SAMPLE.+\d\d$/, #file;