txt = Regex.Replace(txt, "^(" + hs + ")(.*)(" + he + ")$??", "<" + hx + ">$2</"+hx + ">", RegexOptions.Multiline);
txt = Regex.Replace(txt, "^(" + hs + ")(.*)", "<" + hx + ">$2</"+hx + ">", RegexOptions.Multiline);
を併用して解決(多分本当は一発変換できる)
#俺の進捗を見てくれ