Обработка вида Unix символа подчеркивания

Вот быстрый способ использования регулярных выражений и замены.

function formatCurrency( number, dp, ts ) {
  var num = parseFloat( number ); //convert to float
  var pw; //for IE
  dp = parseInt( dp, 10 ); //decimal point
  dp = isNaN( dp ) ? 2 : dp; //default 2 decimal point
  ts = ts || ','; //thousands separator

  return num != number ? 
    false : //return false for NaN
    ( ( 0.9 ).toFixed( 0 ) == '1' ? //for cater IE toFixed bug
        num.toFixed( dp ) : //format to fix n decimal point with round up
        ( Math.round( num * ( pw = Math.pow( 10, dp ) || 1 ) ) / pw ).toFixed( dp ) //for fix ie toFixed bug on round up value like 0.9 in toFixed
    ).replace( /^(-?\d{1,3})((\d{3})*)(\.\d+)?$/, function( all, first, subsequence, dmp, dec ) { //separate string into different parts
      return ( first || '' ) + subsequence.replace( /(\d{3})/g, ts + '$1' ) + ( dec || '' ); //add thousands seperator and re-join all parts
    } );
}