Why not positional variation?

[oshfdk]

Note that the computation made by this script is not accurate for the characters already included in the search string, especially if the search string is very common. E.g., for a string like "or." the computed conditional probability of a preceding "r" (that would produce "ror.") will be lower than the correct value, since the algorithm doesn't account for r's already present in the search string when computing the total number of available r's.