sepThousands.js

// A Javascript version of Allan Odgaard's thousands separator regex
// http://blog.macromates.com/2007/recursion-in-regular-expressions/

123456789.toString().replace(/(\d{1,3})(?=(\d{3})+(?!\d))/g, '$1,');
// => "123,456,789"

A WILD ONE-UP APPEARS:

number.to_s.split('.').tap { it[0] = it[0].reverse.scan(/\d{1,3}/).join(',').reverse }.join('.')

(It’s shorter without the radix-point float support: number.to_s.reverse.scan(/\d{1,3}/).join(',').reverse)

And now, homework: Make it automatically handle internationalization! :D

only problem: it's the wrong language! ;)

Pfft, ‘wrong’ language. Ruby is delicious. Mind you, JavaScript and C are my main languages nowadays, but I still think my Ruby solution is pretty sexy ;D