prex rpex perx eprx repx erpx prxe rpxe pxre xpre rxpe xrpe pexr epxr pxer xper expr xepr rexp erxp rxep xrep exrp xerp
Note: these 'words' (valid or invalid) are all the permutations of the word prex. These words are obtained by scrambling the letters in prex.