Shell is the interface between the user and the operating system. It acts as a keyboard interpreter, taking the keyboard input from the user and delivering it to the operating system. You can think of the shell as being the part of the operating system that allows you to interact with the kernel.

POSIX or "Portable Operating System Interface for Unix" is a collection of standards that define some of the functionality that a UNIX operating system should support. One of these standards defines two flavours of regular expressions. Commands involving regular expressions, such as grep and egrep , implement these flavours on POSIX-compliant UNIX systems.