might be faster to just loop (5 times) through the next two characters in the string and test for equality. lookahead/lookbehind assertions are rather expensive if the string being matched is long 
$matched = undef;
for($index=0,$index<11;$index+=2) {
  if(substr($test_string;$index,2) eq "ab") {
    $matched = 1;
    last;
  }
}

[download]
untested code
the hardest line to type correctly is: stty erase ^H

In reply to Re: regex for negating a sequence by aquarium
in thread regex for negating a sequence by spurperl

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