快速解决方案是检查以前的LA(-1)令牌是否不相等.并添加前导的可选.

Quick fix is to check the previous LA(-1) token if it is unequal . and add a leading optional DOT.

结果语法为:

grammar Test;
words: (WORD|SPACE|DOT)+;
WORD : DOT? (
       LD
       |{_input.LA(-1)!='.'}? DOT       
       ) +        ;
DOT: '.';
SPACE: ' ';
fragment LD: ~[.\n\r ];

玩得开心，享受ANTLR，这是一个很好的工具.

Have fun and enjoy ANTLR, it is a nice tool.