AntlrWorks报错：antlr4及规则‘X’无起始状态问题求助

Hey there! Let's work through your ANTLR problem together—since you're just starting out, that "start state not found" error is super common, and we can fix it while building a grammar that fits your needs.

First, let's unpack that error: The "Cannot display rule 'X' because start state not found" message almost always means ANTLR can't properly resolve your grammar's starting rule. This could happen if:

• You didn't define a clear starting rule (ANTLR defaults to the first rule in your grammar, but if that rule has syntax errors, it can't generate the state machine)
• You misspelled the rule name you're trying to use as the start point
• Your rule has invalid syntax that breaks ANTLR's ability to parse it

Now, let's build a sample grammar that aligns with your goal: finding keywords, building a tree from the numbers/words that follow them, ending with \n, and excluding the | character. Here's a working starting point:

grammar KeywordTree;

// Explicit starting rule—this tells ANTLR where to begin parsing
start
    : keywordLine+ EOF
    ;

// A single line: keyword followed by numbers/words, ending with newline
keywordLine
    : KEYWORD (WORD | NUMBER)+ NEWLINE
    ;

// Replace these with your actual target keywords
KEYWORD
    : 'FIND' | 'PROCESS' | 'BUILD'
    ;

// Match letters/underscores for words
WORD
    : [a-zA-Z_]+
    ;

// Match integers or decimals
NUMBER
    : [0-9]+ ('.' [0-9]+)?
    ;

NEWLINE
    : '\n'
    ;

// Skip the | character and whitespace (except newlines, which we care about)
SKIP_CHARS
    : ('|' | [ \t]) -> skip
    ;

A quick breakdown of this grammar:

• We explicitly define start as the top-level rule, so ANTLR knows exactly where to start (no more "start state not found" issues here)
• keywordLine handles each entry: a keyword, followed by any number of words/numbers, ending with a newline
• The SKIP_CHARS rule automatically ignores any | symbols or spaces/tabs, so they don't interfere with parsing
• All lexer rules (uppercase names) define the basic tokens we need

To test this:

1. Generate the parser with antlr4 KeywordTree.g4
2. Compile the Java files (if using Java) with javac KeywordTree*.java
3. Test with an input like:

   FIND apple 123\n
   PROCESS 45.67 banana\n
   

   Using the command grun KeywordTree start -tree to see the generated parse tree

If you're still hitting issues, share your full grammar code—small syntax mistakes (like missing +/* quantifiers, or mismatched parentheses) can cause that start state error too.

内容的提问来源于stack exchange，提问作者Steve