Best way to remove thousand separators from string amount using a regex

This one may suit your needs:

,(?=[\d,]*\.\d{2}\b)

Debuggex Demo

if (string.match(/\.\d{2}$/) {
    string = string.replace(',', '');
}

or

string.replace(/,(?=.*\.\d+)/g, '');

Replace /,(?=\d*[\.,])/g with empty string?

http://regexr.com/39v2m