Sorry but unless you perform a check at each increment, this incrementing does not prove good. Your implementation just checks the first and last characters only. This will return success if 
 $find = "CDED";
# or
$find = "CDEH";
# or
$find = "CDEJ";
#or even 
$find = "CDEC";

[download]
Also if you check the pattern closely,
BCDEFGHIJKLMNOPCDEFQRST
you can find a CDEF in-between the P and Q. So incrementing the string from $first..$last should not be the approach here.
I believe by "incrementing" what OP meant was incrementing the search string from first to last position. Thanks..

In reply to Re^4: doubt in string matching. by Mandrake
in thread doubt in string matching. by heidi

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