Format a string using regex in Java

Question

Is there any way I can format a string into a specific pattern using regex or is stringbuilder + substring a faster approach?

For example, say a phone number --> 123

Answer 1

StringBuilder with substring will be faster, but not always the simplest/best approach. In this case I would just use substring.

String num = "1234567890";
String formatted = "(" + num.substring(0,3) + ") "
     + num.substring(3,6) + "-" + num.substring(6);