Add commas or spaces to group every three digits

I have a function to add commas to numbers:

function commafy( num ) {
  num.toString().replace( /\B(?=(?:\d{3})+)$/g, "," );
}

Unfortunately, it doesn't like decimals very well. Given the following usage examples, what is the best way to extend my function?

commafy( "123" )                 // "123"
commafy( "1234" )                // "1234"
                                 // Don't add commas until 5 integer digits
commafy( "12345" )               // "12,345"
commafy( "1234567" )             // "1,234,567"
commafy( "12345.2" )             // "12,345.2"
commafy( "12345.6789" )          // "12,345.6789"
                                 // Again, nothing until 5
commafy( ".123456" )             // ".123 456"
                                 // Group with spaces (no leading digit)
commafy( "12345.6789012345678" ) // "12,345.678 901 234 567 8"

Presumably the easiest way is to first split on the decimal point (if there is one). Where best to go from there?