为什么正则表达式“ //”和“ / *”不能匹配单个注释和块注释？

Question

I want to calculate the "empty line","single comment","block comment" about c++ program.

I write the tool use flex.But the tool can't match the c++ block comment.

1 flex code:

%{
    int block_flag = 0;
    int empty_num = 0;
    int single_line_num = 0;
    int block_line_num = 0;
    int line = 0;
%}

%%
^[\t ]*\n               {
    empty_num++;
    printf("empty line\n");
}
"//"    {
    single_line_num++;
    printf("single line comment\n");
}
"/*"  {
    block_flag = 1;
    block_line_num++;
    printf("block comment begin.block line:%d\n", block_line_num);
}

"*/"  {
    block_flag = 0;
    printf("block comment end.block line:%d\n", block_line_num);
}
^(.*)\n                 {
    if(block_flag)
    block_line_num++;
    else
    line++;
}

%%
int main(int argc , char *argv[])
{
    yyin = fopen(argv[1], "r");
    yylex();

    printf("lines :%d\n" ,line);
    fclose(yyin);

    return 0;
}

2 hello.c

bbg@ubuntu:~$ cat hello.c 
#include <stdlib.h>

//
//
/*
 */

/*   */

3 output

bbg@ubuntu:~$ ./a.out hello.c 
empty line
empty line
lines :6

Why the "//" and "/*" can't match the single comment and block comment ?

Answer 1

Flex:

1. doesn't search. It matches patterns sequentially, each one starting where the other one ends.

2. always picks the pattern with the longest match. (If two or more patterns match exactly the same amount, it picks the first one.

So, you have

"//"   { /* Do something */ } 

and

^.*\n  { /* Do something else */ }

Suppose it has just matched the second one, so we're at the beginning of a line, and suppose the line starts // . Now, both these patterns match, but the second one matches the whole line, whereas the first one only matches two characters. So the second one wins. That wasn't what you wanted.

Hint 1: You probably want // comments to match to the end of the line

Hint 2: There is a regular expression which will match /* comments, although it's a bit tedious: "/*"[^*]*"*"+([^*/][^*]*"*"+)*"/" Unfortunately, if you use that, it won't count line ends for you, but you should be able to adapt it to do what you want.

Hint 3: You might want to think about comments which start in the middle of a line, possibly having been indented. You rule ^.*\\n will swallow an entire line without even looking to see if there is a comment somewhere inside it.

Hint 4: String literals hide comments.