Why use a regex at all? 
# perl
$formatted = sprintf("%.2f", $unformatted);

# javascript, assuming it doesn't have a s?printf() function
formatted = Math.round(unformatted * 100) / 100;

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In reply to Re: Using Regular Expressions to Simulate sprintf() function - One Liner Challenge by Fastolfe
in thread Using Regular Expressions to Simulate sprintf() function - One Liner Challenge by Incognito

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