I'd write that as (untested): 
s/^hello kitty \K0*([1-9][0-9]*)(?=x[0-9])/$1/;

[download]
It seem that all you want to do is remove leading 0s.

In reply to Re: regexp: non-capturing grouping in replacement possible? by JavaFan
in thread regexp: non-capturing grouping in replacement possible? by diweooy

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