Replace sting with substing with regular expression : I am not good in regular expression please help to solve this problem. its urgent that's why I can't study more about it.

Problem :

If I have a sting like this `100,000` some digits with `,` followed by some other digits. I need to replace all digits so that the sting become `100`.

I Have to user 'replace()' function so please provide the expression.

    var str = "1200,00";
    str.replace("Expression");// Need this.

Thanks In advance.

    