Dear monks, I saw the question Regexp for alphabetical order match within the string this morning. So I decided to give it a try, and came up with the following solution with matching time interpolation (??{ ... }) inside the regular expression -

use strict;

my $seq1 = 'aabcdefg';
my $seq2 = 'aabcdefgabcde';

sub in_sequence
{
    my $str = shift;  # fetch the string value
    my $ok = 1;       # set private variable $ok = 1
    $str =~ m/(.)(.)(??{ $ok = 0 if $2 lt $1; 0 })/;
    return $ok;
}

print "Sequence is ", in_sequence($seq1)?"good":"bad", "\n";
print "Sequence is ", in_sequence($seq2)?"good":"bad", "\n";

[download]
What I want to achieve in subroutine in_sequence is as follows:
  • get the string value;
  • assign value 1 to my variable $ok;
  • in the regular expression, I fetch two characters at a time, and set $ok to 0 if the 2nd character is less than the 1st character;
  • return the value of $ok to the caller.

    However I am not getting the expected result. The values returned by the subroutine for $str1 and $str2 are both true.

    So I expanded the regular expression and inserted a few print statements. The debug version of the code is as follows:
    use strict;

my $seq1 = 'aabcdefg';
my $seq2 = 'aabcdefgabcde';

sub in_sequence_debug
{
    my $str = shift;
    my $ok = 1;
    print "Value of OK is $ok\n";
    $str =~ m/(.)(.)(??{
                     print "$1 - $2 ";
                     if ($2 lt $1) {
                       $ok = 0;
                       print "Not Ok\n"
                     } else {
                       print "Ok\n";
                     };
                     0
                })/;
    print "Value of OK is $ok\n";
    return $ok;
}

print "Sequence is ", in_sequence_debug($seq1)?"good":"bad", "\n";
print "-"x40, "\n";
print "Sequence is ", in_sequence_debug($seq2)?"good":"bad", "\n";

    [download]
    When I ran the code, I got the following output:
    Value of OK is 1
a - a Ok
a - b Ok
b - c Ok
c - d Ok
d - e Ok
e - f Ok
f - g Ok
Value of OK is 1
Sequence is good
----------------------------------------
Value of OK is 1
a - a Ok
a - b Ok
b - c Ok
c - d Ok
d - e Ok
e - f Ok
f - g Ok
g - a Not Ok
a - b Ok
b - c Ok
c - d Ok
d - e Ok
Value of OK is 1
Sequence is good

    [download]
    It seems that the value of my $ok has not been modified by the code $ok = 0 inside (??{ ... }).

    This suddenly made me curious and created the following test code to investigate the scoping of the variable $ok -
    use strict;

my $seq1 = 'aabcdefg';
my $seq2 = 'aabcdefgabcde';
my $ok; # moved up to the file scope

sub in_sequence_debug
{
    my $str = shift;
    $ok = 1;
    print "Value of OK is $ok\n";
    $str =~ m/(.)(.)(??{
                     print "$1 - $2 ";
                     if ($2 lt $1) {
                       $ok = 0;
                       print "Not Ok\n"
                     } else {
                       print "Ok\n";
                     };
                     0
                })/;
    print "Value of OK is $ok\n";
    return $ok;
}

print "Sequence is ", in_sequence_debug($seq1)?"good":"bad", "\n";
print "-"x40, "\n";
print "Sequence is ", in_sequence_debug($seq2)?"good":"bad", "\n";

    [download]
    And this time, the output becomes -
    Value of OK is 1
a - a Ok
a - b Ok
b - c Ok
c - d Ok
d - e Ok
e - f Ok
f - g Ok
Value of OK is 1
Sequence is good
----------------------------------------
Value of OK is 1
a - a Ok
a - b Ok
b - c Ok
c - d Ok
d - e Ok
e - f Ok
f - g Ok
g - a Not Ok
a - b Ok
b - c Ok
c - d Ok
d - e Ok
Value of OK is 0
Sequence is bad

    [download]
    It actually worked when I move the $ok variable up one level to the file scope!

    It seems that something odd is happing to the variable scoping inside the match-time interpolation (??{ ... }), the variable assignment wrapped inside has no effect on my variables inside the subroutine, probably creates its own private variable, even when I do not explicitly declare them with my.

    Can anyone please explain to me why, by moving my $ok up one level into file scope, made the assignment $ok = 0 inside (??{ ... }) to work all the sudden?

    Thanks in advance!

    In reply to Variable scoping oddity inside (??{ ... }) by Roger

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