Grammar and;

Lexer
e1 = ('a'|'b')+;
e2 = ('a'|'c')+;

t1 = e1 Except e2; // any sequence of `a` and `b` with at least a `b`
t2 = e2 Except e1; // any sequence of `a` and `c` with at least a `c`
t3 = e1 And e2; // 'a'+

Parser

p = q+;
q = t1|t2|t3;