java - 使用可为空/可选标记时进入下一个循环迭代-6ren

java - 使用可为空/可选标记时进入下一个循环迭代

转载作者：行者123 更新时间：2023-11-30 01:57:19

我必须编写要在java应用程序中使用的解析器，它将接受:

数字(例如:1 2 3)
数字范围(例如:1-3)
命名范围(例如:GROUP_1_MATCHED)

每个标记由以下任一分隔:

<WHITE : ([" ", "\t"])+ >
<COMMA : (",") >
<SEMICOLON : (";") >
<EOL : ("\r" | "\n" | "\r\n") >

如果范围没有可选空格，一切都会很容易，例如:

测试字符串是这样的:"1 2 3 4 5,6,7;8;9,, 10;11;;, ;,;,,;\n\n ;,,; 12,13-13, 14 - 14 15- 15 16 -16\n17-17\n 18 - 18\n 19 - 19\n GROUP_1_A;GROUP_1_A GROUP_1_A;GROUP_1_A,GROUP_1_A ,;;\n\n\"GROUP_1_A\";; 20"

我尝试了几种定义“-”周围空格的方法，但总而言之，要么以无限嵌套循环结束，该循环处理给定的简单字符串直到结束，然后从头开始，要么无法进入下一次迭代。如果有一种方法可以检查访问下一个 token 而不消耗它，那就很容易了。

SKIP: {
    < QUOTATION :  ( ["\""] ) > |
    < APOSTROPHE : ( ["'"] ) >
}

TOKEN: {
    < NAME :            ( ["a"-"z", "A"-"Z"])+ (["a"-"z", "A"-"Z", "_", "0"-"9"] )* > |
    < NUM :             ( ["0"-"9"] ){1,5} > |
    < WHITE :           ( [" ", "\t"] ) > |
    < EOL :             ( "\n" | "\r" | "\r\n" ) > |
    < COMMA :           ( [","] ) > |
    < SEMICOLON :       ( [";"] ) >
}

Map<String, List<String>> parse() : {
    Map<String, List<String>> result = new HashMap<String, List<String>>();
    List<String> single = new ArrayList<String>();
    List<String> range = new ArrayList<String>();
    List<String> named = new ArrayList<String>();
    result.put(SINGLE, single);
    result.put(RANGE, range);
    result.put(NAMED, named);
    Token name = null;
    Token first = null;
    Token last = null;
}
{
    (<WHITE>)*
    (
        (name = <NAME> |
            first = <NUM>
            (LOOKAHEAD(2) (<WHITE>)* "-" (<WHITE>)* last = <NUM>)?
        )
        ((LOOKAHEAD(2) <EOL> | <COMMA> | <SEMICOLON> | <WHITE>)+ | <EOF>)

        {
            if (name != null) {
                named.add(name.image);
            } else if (first != null && last == null) {
                single.add(first.image);
            } else if (first != null && last != null) {
                String s = first.image + " - " + last.image;
                range.add(s);
            } else {
                System.err.println("Parser error found");
            }

            name = null;
            first = null;
            last = null;
        }
    )+
    {
        return result;
    }
}

这是解析的输出:

Call:   parse
  Consumed token: <<WHITE>: " " at line 1 column 1>
  Consumed token: <<WHITE>: " " at line 1 column 2>
  Consumed token: <<NUM>: "1" at line 1 column 3>
  Visited token: <<WHITE>: " " at line 1 column 4>; Expected token: <<WHITE>>
  Visited token: <<NUM>: "2" at line 1 column 5>; Expected token: <<WHITE>>
  Visited token: <<NUM>: "2" at line 1 column 5>; Expected token: <"-">
  Visited token: <<WHITE>: " " at line 1 column 4>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 1 column 4>
  Consumed token: <<NUM>: "2" at line 1 column 5>
  Visited token: <<WHITE>: " " at line 1 column 6>; Expected token: <<WHITE>>
  Visited token: <<NUM>: "3" at line 1 column 7>; Expected token: <<WHITE>>
  Visited token: <<NUM>: "3" at line 1 column 7>; Expected token: <"-">
  Visited token: <<WHITE>: " " at line 1 column 6>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 1 column 6>
  Consumed token: <<NUM>: "3" at line 1 column 7>
  Visited token: <<WHITE>: " " at line 1 column 8>; Expected token: <<WHITE>>
  Visited token: <<NUM>: "4" at line 1 column 9>; Expected token: <<WHITE>>
  Visited token: <<NUM>: "4" at line 1 column 9>; Expected token: <"-">
  Visited token: <<WHITE>: " " at line 1 column 8>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 1 column 8>
  Consumed token: <<NUM>: "4" at line 1 column 9>
  Visited token: <<WHITE>: " " at line 1 column 10>; Expected token: <<WHITE>>
  Visited token: <<NUM>: "5" at line 1 column 11>; Expected token: <<WHITE>>
  Visited token: <<NUM>: "5" at line 1 column 11>; Expected token: <"-">
  Visited token: <<WHITE>: " " at line 1 column 10>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 1 column 10>
  Consumed token: <<NUM>: "5" at line 1 column 11>
  Visited token: <<COMMA>: "," at line 1 column 12>; Expected token: <<WHITE>>
  Visited token: <<COMMA>: "," at line 1 column 12>; Expected token: <"-">
  Visited token: <<COMMA>: "," at line 1 column 12>; Expected token: <<EOL>>
  Consumed token: <<COMMA>: "," at line 1 column 12>
  Consumed token: <<NUM>: "6" at line 1 column 13>
  Visited token: <<COMMA>: "," at line 1 column 14>; Expected token: <<WHITE>>
  Visited token: <<COMMA>: "," at line 1 column 14>; Expected token: <"-">
  Visited token: <<COMMA>: "," at line 1 column 14>; Expected token: <<EOL>>
  Consumed token: <<COMMA>: "," at line 1 column 14>
  Consumed token: <<NUM>: "7" at line 1 column 15>
  Visited token: <<SEMICOLON>: ";" at line 1 column 16>; Expected token: <<WHITE>>
  Visited token: <<SEMICOLON>: ";" at line 1 column 16>; Expected token: <"-">
  Visited token: <<SEMICOLON>: ";" at line 1 column 16>; Expected token: <<EOL>>
  Consumed token: <<SEMICOLON>: ";" at line 1 column 16>
  Consumed token: <<NUM>: "8" at line 1 column 17>
  Visited token: <<SEMICOLON>: ";" at line 1 column 18>; Expected token: <<WHITE>>
  Visited token: <<SEMICOLON>: ";" at line 1 column 18>; Expected token: <"-">
  Visited token: <<SEMICOLON>: ";" at line 1 column 18>; Expected token: <<EOL>>
  Consumed token: <<SEMICOLON>: ";" at line 1 column 18>
  Consumed token: <<NUM>: "9" at line 1 column 19>
  Visited token: <<COMMA>: "," at line 1 column 20>; Expected token: <<WHITE>>
  Visited token: <<COMMA>: "," at line 1 column 20>; Expected token: <"-">
  Visited token: <<COMMA>: "," at line 1 column 20>; Expected token: <<EOL>>
  Consumed token: <<COMMA>: "," at line 1 column 20>
  Visited token: <<COMMA>: "," at line 1 column 21>; Expected token: <<EOL>>
  Consumed token: <<COMMA>: "," at line 1 column 21>
  Visited token: <<WHITE>: " " at line 1 column 22>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 1 column 22>
  Visited token: <<WHITE>: " " at line 1 column 23>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 1 column 23>
  Consumed token: <<NUM>: "10" at line 1 column 24>
  Visited token: <<SEMICOLON>: ";" at line 1 column 26>; Expected token: <<WHITE>>
  Visited token: <<SEMICOLON>: ";" at line 1 column 26>; Expected token: <"-">
  Visited token: <<SEMICOLON>: ";" at line 1 column 26>; Expected token: <<EOL>>
  Consumed token: <<SEMICOLON>: ";" at line 1 column 26>
  Consumed token: <<NUM>: "11" at line 1 column 27>
  Visited token: <<SEMICOLON>: ";" at line 1 column 29>; Expected token: <<WHITE>>
  Visited token: <<SEMICOLON>: ";" at line 1 column 29>; Expected token: <"-">
  Visited token: <<SEMICOLON>: ";" at line 1 column 29>; Expected token: <<EOL>>
  Consumed token: <<SEMICOLON>: ";" at line 1 column 29>
  Visited token: <<SEMICOLON>: ";" at line 1 column 30>; Expected token: <<EOL>>
  Consumed token: <<SEMICOLON>: ";" at line 1 column 30>
  Visited token: <<COMMA>: "," at line 1 column 31>; Expected token: <<EOL>>
  Consumed token: <<COMMA>: "," at line 1 column 31>
  Visited token: <<WHITE>: " " at line 1 column 32>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 1 column 32>
  Visited token: <<WHITE>: " " at line 1 column 33>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 1 column 33>
  Visited token: <<SEMICOLON>: ";" at line 1 column 34>; Expected token: <<EOL>>
  Consumed token: <<SEMICOLON>: ";" at line 1 column 34>
  Visited token: <<COMMA>: "," at line 1 column 35>; Expected token: <<EOL>>
  Consumed token: <<COMMA>: "," at line 1 column 35>
  Visited token: <<SEMICOLON>: ";" at line 1 column 36>; Expected token: <<EOL>>
  Consumed token: <<SEMICOLON>: ";" at line 1 column 36>
  Visited token: <<COMMA>: "," at line 1 column 37>; Expected token: <<EOL>>
  Consumed token: <<COMMA>: "," at line 1 column 37>
  Visited token: <<COMMA>: "," at line 1 column 38>; Expected token: <<EOL>>
  Consumed token: <<COMMA>: "," at line 1 column 38>
  Visited token: <<SEMICOLON>: ";" at line 1 column 39>; Expected token: <<EOL>>
  Consumed token: <<SEMICOLON>: ";" at line 1 column 39>
  Visited token: <<EOL>: "\n" at line 1 column 40>; Expected token: <<EOL>>
  Consumed token: <<EOL>: "\n" at line 1 column 40>
  Visited token: <<EOL>: "\n" at line 2 column 1>; Expected token: <<EOL>>
  Consumed token: <<EOL>: "\n" at line 2 column 1>
  Visited token: <<WHITE>: " " at line 3 column 1>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 3 column 1>
  Visited token: <<WHITE>: " " at line 3 column 2>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 3 column 2>
  Visited token: <<WHITE>: " " at line 3 column 3>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 3 column 3>
  Visited token: <<SEMICOLON>: ";" at line 3 column 4>; Expected token: <<EOL>>
  Consumed token: <<SEMICOLON>: ";" at line 3 column 4>
  Visited token: <<COMMA>: "," at line 3 column 5>; Expected token: <<EOL>>
  Consumed token: <<COMMA>: "," at line 3 column 5>
  Visited token: <<COMMA>: "," at line 3 column 6>; Expected token: <<EOL>>
  Consumed token: <<COMMA>: "," at line 3 column 6>
  Visited token: <<SEMICOLON>: ";" at line 3 column 7>; Expected token: <<EOL>>
  Consumed token: <<SEMICOLON>: ";" at line 3 column 7>
  Visited token: <<WHITE>: " " at line 3 column 8>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 3 column 8>
  Visited token: <<WHITE>: " " at line 3 column 9>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 3 column 9>
  Consumed token: <<NUM>: "12" at line 3 column 10>
  Visited token: <<COMMA>: "," at line 3 column 12>; Expected token: <<WHITE>>
  Visited token: <<COMMA>: "," at line 3 column 12>; Expected token: <"-">
  Visited token: <<COMMA>: "," at line 3 column 12>; Expected token: <<EOL>>
  Consumed token: <<COMMA>: "," at line 3 column 12>
  Consumed token: <<NUM>: "13" at line 3 column 13>
  Visited token: <"-" at line 3 column 15>; Expected token: <<WHITE>>
  Visited token: <"-" at line 3 column 15>; Expected token: <"-">
  Visited token: <<NUM>: "13" at line 3 column 16>; Expected token: <<WHITE>>
  Visited token: <<NUM>: "13" at line 3 column 16>; Expected token: <<NUM>>
  Consumed token: <"-" at line 3 column 15>
  Consumed token: <<NUM>: "13" at line 3 column 16>
  Visited token: <<COMMA>: "," at line 3 column 18>; Expected token: <<EOL>>
  Consumed token: <<COMMA>: "," at line 3 column 18>
  Visited token: <<WHITE>: " " at line 3 column 19>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 3 column 19>
  Visited token: <<WHITE>: " " at line 3 column 20>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 3 column 20>
  Consumed token: <<NUM>: "14" at line 3 column 21>
  Visited token: <<WHITE>: " " at line 3 column 23>; Expected token: <<WHITE>>
  Visited token: <<WHITE>: " " at line 3 column 24>; Expected token: <<WHITE>>
  Consumed token: <<WHITE>: " " at line 3 column 23>
  Consumed token: <<WHITE>: " " at line 3 column 24>
  Consumed token: <"-" at line 3 column 25>
  Consumed token: <<WHITE>: " " at line 3 column 26>
  Consumed token: <<WHITE>: " " at line 3 column 27>
  Consumed token: <<WHITE>: " " at line 3 column 28>
  Consumed token: <<WHITE>: " " at line 3 column 29>
  Consumed token: <<NUM>: "14" at line 3 column 30>
  Visited token: <<WHITE>: " " at line 3 column 32>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 3 column 32>
  Consumed token: <<NUM>: "15" at line 3 column 33>
  Visited token: <"-" at line 3 column 35>; Expected token: <<WHITE>>
  Visited token: <"-" at line 3 column 35>; Expected token: <"-">
  Visited token: <<WHITE>: " " at line 3 column 36>; Expected token: <<WHITE>>
  Consumed token: <"-" at line 3 column 35>
  Consumed token: <<WHITE>: " " at line 3 column 36>
  Consumed token: <<NUM>: "15" at line 3 column 37>
  Visited token: <<WHITE>: " " at line 3 column 39>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 3 column 39>
  Consumed token: <<NUM>: "16" at line 3 column 40>
  Visited token: <<WHITE>: " " at line 3 column 42>; Expected token: <<WHITE>>
  Visited token: <"-" at line 3 column 43>; Expected token: <<WHITE>>
  Visited token: <"-" at line 3 column 43>; Expected token: <"-">
  Consumed token: <<WHITE>: " " at line 3 column 42>
  Consumed token: <"-" at line 3 column 43>
  Consumed token: <<NUM>: "16" at line 3 column 44>
  Visited token: <<WHITE>: " " at line 3 column 46>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 3 column 46>
  Visited token: <<EOL>: "\n" at line 3 column 47>; Expected token: <<EOL>>
  Consumed token: <<EOL>: "\n" at line 3 column 47>
  Consumed token: <<NUM>: "17" at line 4 column 1>
  Visited token: <"-" at line 4 column 3>; Expected token: <<WHITE>>
  Visited token: <"-" at line 4 column 3>; Expected token: <"-">
  Visited token: <<NUM>: "17" at line 4 column 4>; Expected token: <<WHITE>>
  Visited token: <<NUM>: "17" at line 4 column 4>; Expected token: <<NUM>>
  Consumed token: <"-" at line 4 column 3>
  Consumed token: <<NUM>: "17" at line 4 column 4>
  Visited token: <<EOL>: "\n" at line 4 column 6>; Expected token: <<EOL>>
  Consumed token: <<EOL>: "\n" at line 4 column 6>
  Visited token: <<WHITE>: " " at line 5 column 1>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 5 column 1>
  Consumed token: <<NUM>: "18" at line 5 column 2>
  Visited token: <<WHITE>: " " at line 5 column 4>; Expected token: <<WHITE>>
  Visited token: <"-" at line 5 column 5>; Expected token: <<WHITE>>
  Visited token: <"-" at line 5 column 5>; Expected token: <"-">
  Consumed token: <<WHITE>: " " at line 5 column 4>
  Consumed token: <"-" at line 5 column 5>
  Consumed token: <<WHITE>: " " at line 5 column 6>
  Consumed token: <<NUM>: "18" at line 5 column 7>
  Visited token: <<EOL>: "\n" at line 5 column 9>; Expected token: <<EOL>>
  Consumed token: <<EOL>: "\n" at line 5 column 9>
  Visited token: <<WHITE>: " " at line 6 column 1>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 6 column 1>
  Consumed token: <<NUM>: "19" at line 6 column 2>
  Visited token: <<WHITE>: " " at line 6 column 4>; Expected token: <<WHITE>>
  Visited token: <"-" at line 6 column 5>; Expected token: <<WHITE>>
  Visited token: <"-" at line 6 column 5>; Expected token: <"-">
  Consumed token: <<WHITE>: " " at line 6 column 4>
  Consumed token: <"-" at line 6 column 5>
  Consumed token: <<WHITE>: " " at line 6 column 6>
  Consumed token: <<NUM>: "19" at line 6 column 7>
  Visited token: <<WHITE>: " " at line 6 column 9>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 6 column 9>
  Visited token: <<EOL>: "\n" at line 6 column 10>; Expected token: <<EOL>>
  Consumed token: <<EOL>: "\n" at line 6 column 10>
  Visited token: <<WHITE>: " " at line 7 column 1>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 7 column 1>
  Consumed token: <<NAME>: "GROUP_1_A" at line 7 column 2>
  Visited token: <<SEMICOLON>: ";" at line 7 column 20>; Expected token: <<EOL>>
  Consumed token: <<SEMICOLON>: ";" at line 7 column 20>
  Consumed token: <<NAME>: "GROUP_1_A" at line 7 column 21>
  Visited token: <<WHITE>: " " at line 7 column 39>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 7 column 39>
  Consumed token: <<NAME>: "GROUP_1_A" at line 7 column 40>
  Visited token: <<SEMICOLON>: ";" at line 7 column 58>; Expected token: <<EOL>>
  Consumed token: <<SEMICOLON>: ";" at line 7 column 58>
  Consumed token: <<NAME>: "GROUP_1_A" at line 7 column 59>
  Visited token: <<COMMA>: "," at line 7 column 77>; Expected token: <<EOL>>
  Consumed token: <<COMMA>: "," at line 7 column 77>
  Consumed token: <<NAME>: "GROUP_1_A" at line 7 column 78>
  Visited token: <<WHITE>: " " at line 7 column 96>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 7 column 96>
  Visited token: <<WHITE>: " " at line 7 column 97>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 7 column 97>
  Visited token: <<COMMA>: "," at line 7 column 98>; Expected token: <<EOL>>
  Consumed token: <<COMMA>: "," at line 7 column 98>
  Visited token: <<SEMICOLON>: ";" at line 7 column 99>; Expected token: <<EOL>>
  Consumed token: <<SEMICOLON>: ";" at line 7 column 99>
  Visited token: <<SEMICOLON>: ";" at line 7 column 100>; Expected token: <<EOL>>
  Consumed token: <<SEMICOLON>: ";" at line 7 column 100>
  Visited token: <<EOL>: "\n" at line 7 column 101>; Expected token: <<EOL>>
  Consumed token: <<EOL>: "\n" at line 7 column 101>
  Visited token: <<EOL>: "\n" at line 8 column 1>; Expected token: <<EOL>>
  Consumed token: <<EOL>: "\n" at line 8 column 1>
  Visited token: <<WHITE>: " " at line 9 column 1>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 9 column 1>
  Visited token: <<WHITE>: " " at line 9 column 2>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 9 column 2>
  Visited token: <<WHITE>: " " at line 9 column 3>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 9 column 3>
  Consumed token: <<NAME>: "GROUP_1_A" at line 9 column 5>
  Visited token: <<WHITE>: " " at line 9 column 24>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 9 column 24>
  Visited token: <<WHITE>: " " at line 9 column 25>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 9 column 25>
  Visited token: <<SEMICOLON>: ";" at line 9 column 26>; Expected token: <<EOL>>
  Consumed token: <<SEMICOLON>: ";" at line 9 column 26>
  Visited token: <<SEMICOLON>: ";" at line 9 column 27>; Expected token: <<EOL>>
  Consumed token: <<SEMICOLON>: ";" at line 9 column 27>
  Visited token: <<WHITE>: " " at line 9 column 28>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 9 column 28>
  Visited token: <<WHITE>: " " at line 9 column 29>; Expected token: <<EOL>>
  Consumed token: <<WHITE>: " " at line 9 column 29>
  Consumed token: <<NUM>: "20" at line 9 column 30>
  Visited token: <<WHITE>: " " at line 9 column 32>; Expected token: <<WHITE>>
  Visited token: <<WHITE>: " " at line 9 column 33>; Expected token: <<WHITE>>
  Consumed token: <<WHITE>: " " at line 9 column 32>
  Consumed token: <<WHITE>: " " at line 9 column 33>
Return: parse

parsers.excel.ParseException: Encountered " <NUM> "1 "" at line 9, column 34.
Was expecting one of:
    <WHITE> ...
    "-" ...

解析器应该产生类似的输出:

single = [1,2,3,4,5,6,7,8,9,10,11,12,20]
range = [13 - 13,14 - 14,15 - 15,16 - 16,17 - 17,18 - 18,19 - 19]
named = [GROUP_1_A,GROUP_1_A,GROUP_1_A,GROUP_1_A,GROUP_1_A,GROUP_1_A]

当解析器不知道空格是来自破折号之前的空格还是来自整个数字的分隔符的空格时，就会出现问题。

如果您知道修改 JavaCC 以正确完成解析提供的字符串的任何方法，我们将不胜感激。

最佳答案

让我们从 JavaCC 退一步，看看您的语法实际上是什么。

parse --> ows ( body )+
body --> part sep
part --> <NAME>
part --> <NUM>
part --> <NUM> ows "-" ows <NUM>
sep --> (<EOL> | <COMMA> | <SEMICOLON> | <WHITE>)+
sep -->  EOF
ows --> (<WHITE>)*

您应该检查一遍，以确保 (a) 我没有犯任何错误，并且 (b) 这确实是您想要的语言。

我不喜欢您处理EOF的方式。它并不是真正的分隔符。我建议使用以下几乎相同的语法

parse --> ows body
body --> part ( sep body | <EOF> )
part --> <NAME>
part --> <NUM>
part --> <NUM> ows "-" ows <NUM>
sep --> (<EOL> | <COMMA> | <SEMICOLON> | <WHITE>)+
ows --> (<WHITE>)*

<小时/>

第一个解决方案:语法前瞻

OP 说如果有一种方法可以检查下一个 token 而不消耗它，那就很容易了。 有。这称为语法前瞻。

我们唯一需要前瞻的地方是区分部分的第二个和第三个产生式。让我们将它们结合起来。

part --> <NAME>
part --> <NUM> ( ows "-" ows <NUM> )?

没有固定长度的前瞻决定是否在第二次产生式中采用可选路径。所以我们使用这样的语法前瞻:

part --> <NAME>
part --> <NUM> ( LOOKAHEAD( ows "-" ) ows "-" ows <NUM> )?

现在，我们完成了。让我们将生成式放回到 JavaCC 中

void parse() : { }
{
    ows() body }
}

void body() : { }
{
    part() ( sep() body()  | <EOF> )
}

void part() : { }
{
   <NAME>
|
   <NUM>
   ( LOOKAHEAD( ows() "-")
     ows() "-" ows() <NUM>
   )?
}

void sep() : {}
{
    (<EOL> | <COMMA> | <SEMICOLON> | <WHITE>)+
}

void ows() : {}
{
    (<WHITE>)*
}

<小时/>

第二个解决方案:LL(1)

我们可以用 LL(1) 语法来解决这个问题吗？是的。让我们回到最初的语法，或者更确切地说，回到将 EOF 移出循环的语法。

parse --> ows body
body --> part (sep body | <EOF>)
part --> <NAME>
part --> <NUM> ( ows "-" ows <NUM> )?
sep --> (<EOL> | <COMMA> | <SEMICOLON> | <WHITE>)+
ows --> (<WHITE>)*

内联part并引入非终结符afternum

parse --> ows body
body --> <NAME> (sep body | <EOF>)
body --> <NUM> afternum
afternum --> ( ows "-" ows <NUM> )? (sep body | <EOF>)
sep --> (<EOL> | <COMMA> | <SEMICOLON> | <WHITE>)+
ows --> (<WHITE>)*

现在问题出在afternum中。

当我们开始解析 afternum 时，有 5 种可能性需要考虑。 (i) 下一个标记是"-"。 (ii) 下一个标记是 EOL、COMMA 或 SEMICOLON。 (iii) 下一个标记是空格。 (iv) 下一个标记是EOF。 (v) 在任何其他情况下，我们都会出错。

对于情况 (ii)，这不能是最后一部分。在情况 (iii) 中，我们刚刚看到的 WHITE 可能是 sep 的第一个字符，也可能会导致连字符。我们创建一个新的非终结符来处理这两种可能性。

afternum --> "-" ows <NUM> (sep body | <EOF>)
afternum --> nonwssep (sep)? body
afternum --> <WHITE> moreafternum
afternum --> EOF

moreafternum --> ows "-" ows <NUM> (sep body | EOF)
               | sep? body

nonwssep --> <EOL> | <COMMA> | <SEMICOLON>

现在问题出在 moreafternum 中，因为如果下一个标记是 WHITE，则任一选择都是可行的。

让我们稍微操作一下moreafternum。目标是公开该 WHITE token ，以便我们可以将其分解出来。

    moreafternum
= By definition
    ows "-" ows <NUM> (sep body | EOF) | sep? body
= Expand the ?
      ows "-" ows <NUM> (sep body | EOF)
    | body
    | sep body
= Expand first `ows` and split white from other cases
      "-" ows <NUM> (sep body | EOF)
    | WHITE ows "-" ows <NUM> (sep body | EOF)
    | body
    | sep body
= Expand the `sep` in the fourth case
      "-" ows <NUM> (sep body | EOF)
    | WHITE ows "-" ows <NUM> (sep body | EOF)
    | body
    | (WHITE | nonwesep) sep? body
= Split the fourth case
      "-" ows <NUM> (sep body | EOF)
    | WHITE ows "-" ows <NUM> (sep body | EOF)
    | body
    | WHITE sep? body
    | nonwssep sep? body
= Duplicate the fourth choice
      "-" ows <NUM> (sep body | EOF)
    | WHITE ows "-" ows <NUM> (sep body | EOF)
    | WHITE sep? body
    | body
    | WHITE sep? body
    | nonwssep sep?
= Combine the second and third choices.
      "-" ows <NUM> (sep body | EOF)
    | WHITE ( ows "-" ows <NUM> (sep body | EOF) | sep? body )
    | body
    | WHITE sep? body
    | nonwssep sep? body
= combine the third, fourth, and fifth choices
      "-" ows <NUM> (sep body | EOF)
    | WHITE ( ows "-" ows <NUM> (sep body | EOF) | sep? body)
    | sep? body
= Definition of moreafternum
      "-" ows <NUM> (sep body | EOF)
    | WHITE moreafternum
    | sep? body

现在我们可以用这个递归版本重新定义moreafternum

moreafternum --> "-" ows <NUM> (sep body | EOF)
               | <WHITE> moreafternum
               | sep? body

如果我们用 JavaCC 编写这个产生式，当下一个标记是 WHITE 时，第二个和第三个选择之间仍然会存在选择冲突。 JavaCC 会优先选择第二个而不是第三个，这正是我们想要的。如果您不喜欢该警告，可以使用 LOOKAHEAD 来抑制它。请注意，此 LOOKAHEAD 不会更改生成的 Java 代码，它只是抑制警告。

void moreafternum() : {} {
       "-" ows() <NUM> (sep() body() | <EOF>)
|
       // LOOKAHEAD( <WHITE> ) // Optional lookahead to suppresss the warning
       <WHITE> moreafternum()
|
       ( sep() )? body() }

通过再次查看 moreafternum，我们可以一直到 LL(1)。

     moreafternum
=  From above
      "-" ows <NUM> (sep body | EOF)
    | WHITE ( ows "-" ows <NUM> (sep body | EOF) | sep? body)
    | body
    | WHITE sep? body
    | nonwssep sep? body
= Fourth choice is subsumed by the second.
      "-" ows <NUM> (sep body | EOF)
    | WHITE ( ows "-" ows <NUM> (sep body | EOF) | sep? body)
    | body
    | nonwssep sep? body
= Combine last two choices
      "-" ows <NUM> (sep body | EOF)
    | WHITE ( ows "-" ows <NUM> (sep body | EOF) | sep? body)
    | (nonwssep sep?)? body
= Original definition of moreaftersep
      "-" ows <NUM> (sep body | EOF)
    | WHITE moreaftersep
    | (nonwssep sep?)? body

总而言之，我们得到

parse --> ows body 

body --> <NAME> (sep body | <EOF>)
body --> <NUM> afternum

afternum --> "-" ows <NUM> (sep body | <EOF>)
afternum --> <WHITE> moreafternum
afternum --> nonwssep (sep)? body
afternum --> EOF

moreafternum --> "-" ows <NUM> (sep body | EOF)
moreafternum --> <WHITE> moreafternum
moreafternum --> ( nonwssep (sep)? )? body

nonwssep --> <EOL> | <COMMA> | <SEMICOLON>

sep --> (nonwssep | <WHITE>)+

ows --> (<WHITE>)*

这是 LL(1)，因此您可以将其转换为 JavaCC，无需先行。

关于java - 使用可为空/可选标记时进入下一个循环迭代，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/53966401/

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java - 使用可为空/可选标记时进入下一个循环迭代