As part of my learning Ruby I am trying to learn how to format strings. 
  The following is an example for formatting a U.S. phone number:

a = "1234567890"
b = "(000) 000-0000"
ai = 0

for i in 0..(b.length) -1
   if b[i,1] == "0"
     b[i,1] = a[ai,1]
     ai += 1
   end
end
puts b

Is there a better (more Rubyish) way to do this?
Obviously I will want to add more code in the future to handle 
exceptions like phone numbers that are not 10 characters long or are 
longer, but this will get me started.