#!/usr/bin/perl -w
use strict;

$_ = '1ab2ab3ab4';
#     ^^^^
#        ^^^^
#           ^^^^
#  three substrings match /\dab\d/

my @nonoverlap = /\dab\d/g;    # this will only find two of them

my @overlap;
while (/(\dab\d)/g) {          # this will find all three
  push(@overlap,$1);
  pos($_) = $-[0] + 1;
}

print "nonoverlap: @nonoverlap\n";    # 1ab2 3ab4
print "overlap:    @overlap\n";       # 1ab2 2ab3 3ab4