How to find the first 6 characters of string in regex?

How to find the first 6 characters of string in regex.

I know the Syntax is to String.substring(strtindex, endindex-1)

For first six characters is:

String test="Welcometothis";

        System.out.println(test.substring(0,7));

But i need the syntax to find the first 6 characters of string in regex using java.