Black magic! I'm really impressed, I didn' know you can put that all into the substitution pattern. /me bows to Nuance ...

As for my solution, I was focused on the sample input given, I didn't think of the more general problem. But you're right about this.

As for your script, it works with a small alteration:

$string =~ s/($pattern)/$i++ == $n ? $better : $1/ge;
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If you increment $i in a seperate statement, the whole block evaluates to the value of $i und you get

abacab
0b1c2b
[download]

Ok, now we're having fun :-)...

Andreas (waiting for the chinese food delivery service ...)