Here you can find a small R package that will allow the user to build an FM index from an input FASTA file. The FM index will then be used to look for patterns using a function that implements backward search.

Creation of the FM index of a FASTA file. Read more: Ben Langmead: “Introduction to the Burrows-Wheeler Transform and FM Index”. https://www.cs.jhu.edu/~langmea/resources/bwt_fm.pdf