Example 2: W3
  - From UAX#9:
 
    - W3: change all ALs to R.
 
  - Actual code:
 
    s/$AL/$R/g; 
  - Details:
 
  s/foo/bar/: Substitution 
  $AL, $R: Variables containing single letters:
    used as macros for character types 
  g: Global substitution: substitute not only once, but as
    many times as you can