The (* >) operator [1] (the space in the middle is just to circumvent HN's rather dumb comment formatting) sequences its two arguments, ignoring the one on the left. So

    string "0x" *> takeWhile1 hexadecimal

parses a literal string "0x", then one or more hexadecimal digits, and the result is just the hexadecimal digits.

[1] http://hackage.haskell.org/packages/archive/base/latest/doc/...