Exercise: Hide substring

Given a string:

• show 4 * characters and then the last 4 characters
• show the first 4 characters and then 4 stars
• replace every character, except the last 4 by a *
• replace every character, except the first 4 by a *

Here the first line is the input, the rest are the 4 output strings:

0123456789
****6789
0123****
******6789
0123******