X251: notReplace

Given a string, return a string where every appearance of the lowercase word "is" has been replaced with "is not". The word "is" should not be immediately preceeded or followed by a letter -- so for example the "is" in "this" does not count. (Note: Character.isLetter(char) tests if a char is a letter.)


notReplace("is test") -> "is not test"
notReplace("is-is") -> "is not-is not"
notReplace("This is right") -> "This is not right"

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