词法分析器中的 ANTLR4 负前瞻 [英] ANTLR4 negative lookahead in lexer

问题描述

我正在尝试为 PostgreSQL SQL 定义词法分析器规则.

I am trying to define lexer rules for PostgreSQL SQL.

问题在于运算符定义和行注释相互冲突.

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

例如 @--- 是操作符标记 @- 后跟 -- 注释而不是操作符标记 @---

for example @--- is an operator token @- followed by the -- comment and not an operator token @---

在 grako 中，可以为 - 片段定义负前瞻，例如:

In grako it would be possible to define the negative lookahead for the - fragment like:

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

在 ANTLR4 中，我找不到任何方法来回滚已经消耗的片段.

In ANTLR4 I could not find any way to rollback already consumed fragment.

有什么想法吗?

这里是 PostgreSQL 运算符的原始定义:

Here the original definition what the PostgreSQL operator can be:

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.

推荐答案

您可以在词法分析器规则中使用语义谓词来执行前瞻(或后视)而不消耗字符.例如，以下内容涵盖了运算符的几个规则.

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) != '*'}?
    )+
  ;

然而，上述规则并未解决在运算符末尾包含 + 或 - 的限制.为了尽可能以最简单的方式处理这个问题，我可能会将这两种情况分成不同的规则.

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)
  ;

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