String#tr

2.6.2 :001 > "foo".tr("o", "O")
 => "fOO" 
2.6.2 :002 > "foo".tr("o", "e")
 => "fee" 
2.6.2 :003 > "foo".tr("a-z", "0-9")
 => "599" 

2.6.2 :004 > "abc".tr("a-z", "0-9")
 => "012" 

:naruhodo:

2.6.2 :010 > "abcdefghijk".tr("a-z", "0-9")
 => "01234567899"