How to detect and print comment line from a source code?

This is a good occasion to learn how a finite state machine works.
Try, for instance, the following code.
It expects its input (C source code with only original C-style (/*...*/) style comments) form stdin (the console), so redirect (using <) the C source file to be processed appropriately).

#include <stdio.h>


/* The states defining the state machine */
typedef enum
{
  Idle, /* outside a comment */
  WaitForStar, /* outside a comment, but a '/' has been just processed */
  InComment, /* inside a comment */
  WaitForSlash, /* inside a comment, but a '*' as been just processed */
} State;

/* the state machine implementation */
State process_idle(int );
State process_wait_for_star(int );
State process_in_comment(int );
State process_wait_for_slash(int );

int main()
{
  State state = Idle;
  int i;

  while ( (i = fgetc(stdin)) != EOF )
  {
    switch (state)
    {
    case Idle:
      state = process_idle(i);
      break;
    case WaitForStar:
      state = process_wait_for_star(i);
      break;
    case InComment:
      state = process_in_comment(i);
      break;
    case WaitForSlash:
      state = process_wait_for_slash(i);
      break;
    }
  }

  return 0;
}

State process_idle(int i)
{
  if ( i == '/') return WaitForStar;
  return Idle;
}

State process_wait_for_star(int i)
{
  if ( i == '*' ) return InComment;
  if ( i == '/' ) return WaitForStar;
  return Idle;
}

State process_in_comment(int i)
{
  if ( i == '*') return WaitForSlash;
  putchar(i);
  return InComment;
}

State process_wait_for_slash(int i)
{
  if (i == '/')
  {
    putchar('\n');
    return Idle;
  }
  if (i == '*')
  {
    putchar('*');
    return WaitForSlash;
  }
  putchar('*'); putchar(i);
  return InComment;
}

Depends on the language you are processing: comments come in many forms, depending on the language they are used in:

//  Single line comment, ends at newline: "modern" C, C++, C#, Java, ...
/* Multiline comment, ends at */ : all C, C++, etc.
' Single line comment, ends at newline: VB
# Single line comment: PHP, Python
C (in column 1): Early Fortran
REM Single line comment, Basic
:: Batch files
; Most assembly languages
-- SQL
*> Cobol

There are probably loads of others, but those are ones I've used!

So decide what constitutes a comment indicator, and parse each line to find them.

Quote:

How to search them from file a short code please.

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