一个TXT文件，内容如下：
...........22221237542221111211111111111111111111111112212212222222222.............
..........2222121902211111221222111112111122112222221212112111111112222............
.........71211111....111111111111111111111122...22212121111221111..211121..........
.........111111111....111111111111111111111118...11111111111111111..1211127........
........1111141112....222111111111111211111111...21111211111111111..11111112.......
.......31111131111....112111111111111111111117...11111111111111111..11121113.......
......61111111111....111221121111112111212111...31111111111111111..211111125.......
......411111116111....112211111111111111111114...11111111111111111..111211224......
.....211111111112....121111111111111111111112...81111111111111212..211121128.......
....11111171111113....366111111111111111111116...11111111111111111..111111221......
....11111111111114231110281111111111111111111111111111111111111112111111121........
....11111111141171587651191111111111111111111111111112111111111111111111111........

现在想把每行中两个数字之间的 . 替换成字母 M ，
两边的 . 不替换，两边的 . 数量也不变，
请教该如何编写这段语句？