I didn't understand s and m switch of regex until furry_marmot's explanation... man perlre says about /ms

'let the "." match any character whatsoever, while still allowing "^" and "$" to match, respectively, just after and just before newlines within the string'

I didn't think of example that needs this. Do you have any example case like 'little princess' example for /ms?

As for block mode of this example, I saw this way in awk script. I first met this way($\='') in perl.

People sometimes say regex is slow, so I tried to use index function insted of regex. But it seems not improving time. I simplified just to pick up From address in this example and index version needs utf8 treatment for index and substr. 
use strict;
use warnings;

use File::Find;
use Data::Dumper;

my %addresses;

sub test1 {
    my ($from);
    find(sub {
        return unless -f $_;

        open my $fh, '<', $_ or die;
            local $/ = '';      # "Paragraph" mode, reads a block of t
+ext to next \n\n
            $_ = <$fh>;         # Read Header block

            ($from)= $_ =~ /^From:(.*)/m; # /m to anchor
            #print "$from\n";

        close $fh;
    }, glob('./009_mailtest/*'));

    #print Dumper \%addresses;

}
sub test2{
    binmode(STDOUT,":utf8");
    my ($from,$bgn,$end,$len);

    find(sub {
        return unless -f $_;

        open my $fh, '<:utf8', $_ or die;
            local $/ = '';      # "Paragraph" mode, reads a block of t
+ext to next \n\n
            $_ = <$fh>;         # Read Header block

            $bgn=index($_,"From:",0) + length("From:");
            $end=index($_,chr(10),$bgn+1);
            $len=$end - $bgn;
            $from=substr($_, $bgn, $len);
            #print "$from\n";
        close $fh;
    }, glob('./009_mailtest/*'));
}

my($start,$end);
$start=(times)[0];
&test1;
$end=(times)[0];
print "with regex=" . ($end - $start) . "sec\n";

$start=(times)[0];
&test2;
$end=(times)[0];
print "without regex=" . ($end - $start) . "sec\n";

[download]
The result for my 319Mb test mail box was like this. 
with regex=0.296875sec
without regex=0.34375sec

In reply to Re^2: Looking for ideas on how to optimize this specialized grep by remiah
in thread Looking for ideas on how to optimize this specialized grep by afresh1

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