Boolean Expression in a Loop's Test

The test part of a loop (any of the three varieties) is a boolean expression. We want a boolean expression that evaluates to true if the user enters:

• yes
• YES
• Y
• y

and evaluates to false for anything else. Here is a near-complete expression:

( chars.equals( "yes" )   chars.equals( "YES" )   
  chars.equals( "y" )     chars.equals( "Y" )  )