Hello Monks,

Please see update below.

While doing pattern matches using qr// with modifiers added, I ran into behaviour I didn’t expect. The following code, using the 's' modifier to

“change "." to match any character whatsoever, even a newline, which normally it would not match.” (perlre: Modifiers)

illustrates what I mean:

#! perl
use strict;
use warnings;

my $text = <<EOT;
The quick brown fox
jumps over the unfortunate dog.
EOT

my %searches  = ( 'A'         =>     'fox.+?jumps',
                  'B'         => '(?s)fox.+?jumps' );

my %functions = ( '1. no'     => \&no_mod,
                  '2. qr//'   => \&qr_mod,
                  '3. late'   => \&late_mod,
                  '4. inline' => \&inline_mod );

foreach my $search_key (sort keys %searches)
{
    print "\nSearching on '$searches{$search_key}':\n\n";

    foreach my $fn_key (sort keys %functions)
    {
        printf "%s%-9s 's' modifier: %s\n",
                $search_key,
                $fn_key,
                $functions{$fn_key}->($text, $searches{$search_key}) ?
+ 'Match found' : 'No match';
    }
}

sub no_mod                              # no 's' modifier
{
    my ($string, $pattern) = @_;
    my  $regex = qr/$pattern/;

    return ($string =~ /$regex/);
}

sub qr_mod
{
    my ($string, $pattern) = @_;
    my  $regex = qr/$pattern/s;         # <= 's' modifier

    return ($string =~ /$regex/);
}

sub late_mod
{
    my ($string, $pattern) = @_;
    my  $regex = qr/$pattern/;

    return ($string =~ /$regex/s);      # <= 's' modifier
}

sub inline_mod
{
    my ($string) = @_;

    return ($string =~ /fox.+?jumps/s); # <= 's' modifier
}

__END__
[download]

This is the output I get:

Searching on 'fox.+?jumps':

A1. no     's' modifier: No match
A2. qr//   's' modifier: Match found
A3. late   's' modifier: No match
A4. inline 's' modifier: Match found

Searching on '(?s)fox.+?jumps':

B1. no     's' modifier: Match found
B2. qr//   's' modifier: Match found
B3. late   's' modifier: Match found
B4. inline 's' modifier: Match found
[download]

(I’m running DWIM/Strawberry perl 5.14.2 on Vista 32-bit, and I get the same result with perl 5.10.1 on Cygwin.)

All the results are as expected, except for A3. I can’t see any (logical) difference between the match patterns in late_mod() and inline_mod(), yet A4 matches (as expected) but A3 does not.

I’ve looked at perlop: Regexp Quote Like Operators, also perlfaq6: I'm having trouble matching over more than one line. What's wrong? and How do I match a regular expression that's in a variable? , , but I haven’t found anything that addresses this particular issue.

So my questions are:

1. Is the match in A3 supposed to fail?
2. If Yes, why?
3. And then, why doesn’t the use warnings pragma result in a warning along the lines of: Useless use of pattern match modifier 's' on line 53 ?
4. If No (i.e., the match should succeed), what am I doing wrong?

Update

Since posting, I’ve found How do I apply switches like /i or /g to a qr regexp? which addresses this issue. But I would still appreciate any further information or clarification.

Thanks,

---

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