Sed is a popular utility that enables quick parsing and transformation of text. Here are some basic uses for it:

Substitute the first occurrence of editor with tool:

$:~/hackerrank/bash/grep/grep1$ echo "My favorite programming editor is Emacs. Another editor I like is Vim." | sed -e s/editor/tool/
My favorite programming tool is Emacs. Another editor I like is Vim.

Substitute all occurrences of editor with tool:

$:~/hackerrank/bash/grep/grep1$ echo "My favorite programming editor is Emacs. Another editor I like is Vim." | sed -e s/editor/tool/g
My favorite programming tool is Emacs. Another tool I like is Vim.  

Substitute the second occurrence of editor with tool:

$:~/hackerrank/bash/grep/grep1$ echo "My favorite programming editor is Emacs. Another editor I like is Vim." | sed -e s/editor/tool/2
My favorite programming editor is Emacs. Another tool I like is Vim.

Highlight all occurrences of editor by enclosing them in curly brackets (i.e., {}):

$:~/hackerrank/bash/grep/grep1$ echo "My favorite programming editor is Emacs. Another editor I like is Vim." | sed -e s/editor/{\&}/g
My favorite programming {editor} is Emacs. Another {editor} I like is Vim.

Task

Given lines of credit card numbers, mask the first digits of each credit card number with an asterisk (i.e., *) and print the masked card number on a new line. Each credit card number consists of four space-separated groups of four digits. For example, the credit card number 1234 5678 9101 1234 would be masked and printed as **** **** **** 1234.

References
You may find the following links helpful in learning about sed:

• Sed: An Introduction and Tutorial
• The TLDP Guide
• Some Practical Examples
• A StackOverflow question on a slightly modified version of this task where the solution involves backreferences.
• A ttuorial from TheGeekStuff detailing the use of groups and backreferences.