What you want to do is globally match a something including possible repetitions, and replace what you've found with that something followed by the length of your match:

$s =~ s/((.)\2*)/$2 . length $1/eg;

— Arien

Edit: It seems I misread the output you want. To only have sequences of two or more repeated letters replaced, change the star to a plus sign. (And after some sleep...) Also, you'd want to swap length $1 and $2 to have the length preceed the letter.

In reply to Re: Regexp: can I do it in one go? by Arien
in thread Regexp: can I do it in one go? by moxliukas

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