I think I got it, there is one \s and it is outside of the lookaround, so it gets consumed (advances pos), and backtracking doesn't go back before it, so its checking every other number

Look at the pos, in your version (\s outside (?=)) it jumps 7/15 but with \s inside (?=) it jumps 7/11/15 

#! perl -slw
use strict;
my $data = '105 106 107 108 109 110 111 112 113
1115 1116 1117 1118 1119 1120 1121 1122 1123
12345 12346 12347 12348 12349 12350 12351 12353
';
{
    my $other = @ARGV ? qr{(?=\s(\d+)\d\s)} : qr{\s(?=(\d+)\d\s)};
    open my($DATA),'<',\$data;
    my $what = "";
    while( readline $DATA ) {
        s{
\s(\d+)\d
\K
$other
    }{
        warn "WHAT($1)($2)POS(@{[pos()]})\n";
        $1 + 1 == $2 ? ";\n" : '! '
    }gex;
        print;
    }
    print "\n$what";
}
__END__
$ perl junk
WHAT(10)(10)POS(7)
WHAT(10)(10)POS(15)
WHAT(11)(11)POS(23)
WHAT(11)(11)POS(31)
105 106! 107 108! 109 110! 111 112! 113

WHAT(111)(111)POS(9)
WHAT(111)(111)POS(19)
WHAT(112)(112)POS(29)
WHAT(112)(112)POS(39)
1115 1116! 1117 1118! 1119 1120! 1121 1122! 1123

WHAT(1234)(1234)POS(11)
WHAT(1234)(1234)POS(23)
WHAT(1235)(1235)POS(35)
12345 12346! 12347 12348! 12349 12350! 12351 12353




$ perl junk lookaround
WHAT(10)(10)POS(7)
WHAT(10)(10)POS(11)
WHAT(10)(10)POS(15)
WHAT(10)(11)POS(19)
WHAT(11)(11)POS(23)
WHAT(11)(11)POS(27)
WHAT(11)(11)POS(31)
105 106!  107!  108!  109;
 110!  111!  112!  113

WHAT(111)(111)POS(9)
WHAT(111)(111)POS(14)
WHAT(111)(111)POS(19)
WHAT(111)(112)POS(24)
WHAT(112)(112)POS(29)
WHAT(112)(112)POS(34)
WHAT(112)(112)POS(39)
1115 1116!  1117!  1118!  1119;
 1120!  1121!  1122!  1123

WHAT(1234)(1234)POS(11)
WHAT(1234)(1234)POS(17)
WHAT(1234)(1234)POS(23)
WHAT(1234)(1235)POS(29)
WHAT(1235)(1235)POS(35)
WHAT(1235)(1235)POS(41)
12345 12346!  12347!  12348!  12349;
 12350!  12351!  12353

[download]

rxrx gave me the idea to pos it

In reply to Re: Why doesn't this regex work? ( lookaround backtracking pos) by Anonymous Monk
in thread Why doesn't this regex work? (Solved!) by BrowserUk

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