irb, Ruby 1.9.1

What am I missing here?

"b T T W b".match(/(?<!t t|a b) w/i)
=> nil

#The second look-behind is now just a
"b T T W b".match(/(?<!t t|a) w/i)
=> #<MatchData " W">

#Regex stays the same, the T T are now in lower case
"b t t W b".match(/(?<!t t|a) w/i)
=> nil

#Look-behind only contains the t t condition now and, T T are back to
upper case
"b T T W b".match(/(?<!t t) w/i)
=> nil

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