Comma separation code in Java

Displaying large figures without comma separation is quite daunting to the users. For an example just think how hard to discern the value of 9501625237.56 ? Instead of displaying such number as it is we can format the said value using commas which would be more readable , like this  9,501,625,237.56 If displayed likewise An user wouldn't have to waste time deciphering such lengthy values. Here is the code I wrote to accomplish this output using Java to utilize into much larger programs. The method can be overridden or overloaded as you would deem necessary. 

Input : Takes the value to be formatted as a  String object.
Returns: Formatted value as a String object.

private String addCommas(String strn){

         String commaSeparatedNumber="", decimals="";
        //split contents before the decimal point

        commaSeparatedNumber = strn.contains(".") ?   strn.split("\\.")[0] : strn;
        decimals = strn.contains(".") ? "."+strn.split("\\.")[1] : "";

        /*create a regular expression for the thousand separator, store those matches separately for the replacement.*/
        String regx="(\\d+)(\\d{3})";

        //create pattern and matcher objects to find matches
        Pattern p = Pattern.compile(regx);
        Matcher m = p.matcher(commaSeparatedNumber);

        //iterate to find the next match.
        while(m.find()){
            //replace the found matching groups with the comma.
            commaSeparatedNumber = commaSeparatedNumber.replaceFirst(regx, m.group(1) +","+ m.group(2));
            //update the matcher with new replacements.
            m = p.matcher(commaSeparatedNumber);
        }

        return commaSeparatedNumber+decimals;
    }