ANTLR4 negative lookahead in lexer

Question

I am trying to define lexer rules for PostgreSQL SQL.

The problem is with the operator definition and the line comments conflicting with each other.

for exam

Answer 1

You can use a semantic predicate in your lexer rules to perform lookahead (or behind) without consuming characters. For example, the following covers several rules for an operator.

OPERATOR
  : ( [+*<>=~!@#%^&|`?]
    | '-' {_input.LA(1) != '-'}?
    | '/' {_input.LA(1) != '*'}?
    )+
  ;

However, the above rule does not address the restrictions on including a + or - at the end of an operator. To handle that in the easiest way possible, I would probably separate the two cases into separate rules.

// this rule does not allow + or - at the end of a rule
OPERATOR
  : ( [*<>=~!@#%^&|`?]
    | ( '+'
      | '-' {_input.LA(1) != '-'}?
      )+
      [*<>=~!@#%^&|`?]
    | '/' {_input.LA(1) != '*'}?
    )+
  ;

// this rule allows + or - at the end of a rule and sets the type to OPERATOR
// it requires a character from the special subset to appear
OPERATOR2
  : ( [*<>=+]
    | '-' {_input.LA(1) != '-'}?
    | '/' {_input.LA(1) != '*'}?
    )*
    [~!@#%^&|`?]
    OPERATOR?
    ( '+'
    | '-' {_input.LA(1) != '-'}?
    )+
    -> type(OPERATOR)
  ;