If you do it this way, you both avoid the problem and the need for capturing parens, so it's more efficient to boot. 

s[(?<=[a-m])(?=[n-z])][5]g;

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"Efficiency is intelligent laziness." -David Dunham
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In reply to Re: Inserting numbers between parenthese matches in regexp by BrowserUk
in thread Inserting numbers between parenthese matches in regexp by Melroch

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