在antlr4中处理可选令牌的最佳方法是什么

假设我有以下输入:

Great University
Graduated in 2010
Some University
09/2009 - 06/2011
Nice University
06/2011

我想应付多年的学习。我的语法是这样的:

education:
    (section)*
    EOF
    ;

section:
    (school | years)+
   ;

degree:     WORD* DEGREE WORD* SEPARATOR;
years:      WORD* ( (YEAR_START '-')? YEAR_END) WORD* SEPARATOR;
WS          : [ \t\r]+ -> skip;
SEPARATOR   : (NEWLINE | COMMA);
COMMA       : ',';
NEWLINE     : '\n';
SCHOOL      : ('university' | 'University' | 'school' | 'School');
WORD        : [a-zA-Z'()]+;
YEAR_START  : YEAR;
YEAR_END    : YEAR;
YEAR        : (DIGIT DIGIT '/')? [1-2] DIGIT DIGIT DIGIT;
DIGIT       : [0-9];

我收到以下错误:

line 1:17 mismatched input '\n' expecting '-'
line 6:17 mismatched input '\n' expecting '-'

如何通过语法处理可选的开始年份?