lexer中的ANTLR4负向前瞻

Question

我试图为PostgreSQL SQL定义lexer规则。

OP_MINUS: '-' ! ( '-' ) .

这里是PostgreSQL运算符的原始定义：

The operator name is a sequence of up to NAMEDATALEN-1
(63 by default) characters from the following list:

 + - * / < > = ~ ! @ # % ^ & | ` ?

There are a few restrictions on your choice of name:
-- and /* cannot appear anywhere in an operator name,
since they will be taken as the start of a comment.

A multicharacter operator name cannot end in + or -,
unless the name also contains at least one of these
characters:

~ ! @ # % ^ & | ` ?

For example, @- is an allowed operator name, but *- is not.
This restriction allows PostgreSQL to parse SQL-compliant
commands without requiring spaces between tokens.

Answer 1

您可以在lexer规则中使用语义谓词来执行lookahead（或behind）而不使用字符。例如，下面介绍了运算符的几个规则。

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

但是，上面的规则没有解决在运算符末尾包含+或-的限制。为了以最简单的方式处理这一问题，我可能会将这两种情况分成不同的规则。

// 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)
  ;