markov[ruleset_, text_] :=
  Module[{terminating = False, output = text,
    rules = StringCases[
      ruleset, {StartOfLine ~~ pattern : Except["\n"] .. ~~
         " " | "\t" .. ~~ "->" ~~ " " | "\t" .. ~~ dot : "" | "." ~~
         replacement : Except["\n"] .. ~~ EndOfLine :> {pattern,
         replacement, dot == "."}}]},
   While[! terminating, terminating = True;
    Do[If[! StringFreeQ[output, rule[[1]]],
      output = StringReplace[output, rule[[1]] -> rule[[2]]];
      If[! rule[[3]], terminating = False]; Break[]], {rule, rules}]];
    output];