Insert commas into number string

If your language supports postive lookahead assertions, then I think the following regex will work:

(\d)(?=(\d{3})+$)

Demonstrated in Java:

import static org.junit.Assert.assertEquals;

import org.junit.Test;

public class CommifyTest {

    @Test
    public void testCommify() {
        String num0 = "1";
        String num1 = "123456";
        String num2 = "1234567";
        String num3 = "12345678";
        String num4 = "123456789";

        String regex = "(\\d)(?=(\\d{3})+$)";

        assertEquals("1", num0.replaceAll(regex, "$1,"));
        assertEquals("123,456", num1.replaceAll(regex, "$1,"));
        assertEquals("1,234,567", num2.replaceAll(regex, "$1,"));
        assertEquals("12,345,678", num3.replaceAll(regex, "$1,"));
        assertEquals("123,456,789", num4.replaceAll(regex, "$1,"));    
    }    
}

Found on http://gskinner.com/RegExr/

Community > Thousands separator

Pattern: /\d{1,3}(?=(\d{3})+(?!\d))/g

Replace: $&,

trace ( String("1000000000").replace( /\d{1,3}(?=(\d{3})+(?!\d))/g , "$&,") );

It done the job!