There are many algorithms to search for a string within another string. This recipe will use an existing breakSubstring function in the Data.ByteString library to do most of the heavy lifting.
The ByteString documentation establishes its merits by declaring the following claim:
"[A ByteString is] a time- and space-efficient implementation of byte vectors using packed Word8 arrays, suitable for high performance use, both in terms of large data quantities, or high speed requirements. Byte vectors are encoded as strict Word8 arrays of bytes, held in a ForeignPtr, and can be passed between C and Haskell with little effort."
More information and documentation can be obtained on the package web page at http://hackage.haskell.org/package/bytestring/docs/Data-ByteString.html.