I'm assuming that the original poster wanted the frequency of all pairs, so we don't want the regex engine to consume more than one character per iteration. With... 
$genome="acgt";

[download]
Then the look ahead version will finish with... 
$count{ac} == 1
$count{cg} == 1  #Note this match
$count{gt} == 1

[download]
...while the non-lookahead version will have... 
$count{ac} == 1
$count{gt} == 1

[download]
... i.e. it misses the "cg" case because it jumps through the string two characters at a time.

In reply to Re: Re: using s/// as map? by sleepingsquirrel
in thread how can I speed up this perl?? by Anonymous Monk

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