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7.8 Yes-or-No Questions

Some non GUI programs ask a yes-or-no question. If the messages (especially the questions) are translated into foreign languages, be sure that you localize the answers too. It would be very bad habit to ask a question in one language and request the answer in another, often English.

The GNU C Library contains rpmatch to give applications easy access to the corresponding locale definitions.

— Function: int rpmatch (const char *response)

The function rpmatch checks the string in response whether or not it is a correct yes-or-no answer and if yes, which one. The check uses the YESEXPR and NOEXPR data in the LC_MESSAGES category of the currently selected locale. The return value is as follows:

1
The user entered an affirmative answer.
0
The user entered a negative answer.
-1
The answer matched neither the YESEXPR nor the NOEXPR regular expression.

This function is not standardized but available beside in the GNU C Library at least also in the IBM AIX library.

This function would normally be used like this: 

       ...
       /* Use a safe default.  */
       _Bool doit = false;
     
       fputs (gettext ("Do you really want to do this? "), stdout);
       fflush (stdout);
       /* Prepare the getline call.  */
       line = NULL;
       len = 0;
       while (getline (&line, &len, stdin) >= 0)
         {
           /* Check the response.  */
           int res = rpmatch (line);
           if (res >= 0)
             {
               /* We got a definitive answer.  */
               if (res > 0)
                 doit = true;
               break;
             }
         }
       /* Free what getline allocated.  */
       free (line);

Note that the loop continues until an read error is detected or until a definitive (positive or negative) answer is read.