Lex：编写正则表达式时在“[”标记之前的预期表达式

Question

我是 lex/yacc 的新手，正在学习本教程：https://www.youtube.com/watch?v=54bo1qaHAfk
这是我的 lex 文件

%{
    #include "main.h"
    #include <stdio.h>
%}

%%
    [a-zA-Z][_a-zA-Z0-9]*   {return IDENTIFIER;}
    "&"                     {return RUN_DAEMON;}
    "|"                     {return SYM_PIPE;}
    ">"                     {return RED_STDOUT;}
    "<"                     {return RED_STDIN;}
    ">>"                    {return APP_STDOUT;}
    [ \t\n]+                {;}
    .                       {printf("unexpected character\n");}
%%

int yywrap(){
    return 1;
}

但是在运行 lex 命令之后我尝试用 gcc 编译 lex.yy.c 它用这个错误向我发送垃圾邮件

sbash.l: In function ‘yylex’:
sbash.l:7:5: error: expected expression before ‘[’ token
     [a-zA-Z][_a-zA-Z0-9]*   {return IDENTIFIER;}
     ^
sbash.l:7:6: error: ‘a’ undeclared (first use in this function)
     [a-zA-Z][_a-zA-Z0-9]*   {return IDENTIFIER;}
      ^
sbash.l:7:6: note: each undeclared identifier is reported only once for each function it appears in
sbash.l:7:14: error: ‘_a’ undeclared (first use in this function)
     [a-zA-Z][_a-zA-Z0-9]*   {return IDENTIFIER;}
              ^~
sbash.l:7:17: error: ‘zA’ undeclared (first use in this function)
     [a-zA-Z][_a-zA-Z0-9]*   {return IDENTIFIER;}
                 ^~
sbash.l:7:20: error: ‘Z0’ undeclared (first use in this function)
     [a-zA-Z][_a-zA-Z0-9]*   {return IDENTIFIER;}
                    ^~
sbash.l:7:29: error: expected expression before ‘{’ token
     [a-zA-Z][_a-zA-Z0-9]*   {return IDENTIFIER;}
                             ^
sbash.l:13:7: error: stray ‘\’ in program
     [ \t\n]+                {;}
       ^
sbash.l:13:9: error: stray ‘\’ in program
     [ \t\n]+                {;}

不幸的是，我什至用谷歌搜索都找不到问题所在（许多示例的表达式与我的代码完全相同）。
我的 lex 版本是 2.6.1，在 CentOS8

Answer 1

如 Flex manual chapter on flex input file format 中所述，模式规则必须从左边距开始：

The rules section of the flex input contains a series of rules of the form:
  pattern   action 
where the pattern must be unindented and the action must begin on the same line. (Some emphasis added)

规则部分的缩进行只是按原样通过。特别是，第一条规则之前的缩进行被插入到 yylex 函数的顶部，这通常很有用。但是 flex 不会尝试验证以这种方式包含的代码是否有效；编译生成的扫描器时会检测到错误。

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