yesno.c

/*
 * yesno.c - Get a yes/no answer from the user
 */

#define _POSIX_C_SOURCE 200809L
#include <langinfo.h>
#include <regex.h>
#include <stdio.h>

/*
 * Return values of yesno()
 */
enum {
	YN_YES = 0,	/* affirmative response */
	YN_NO = 1,	/* negative response */
	YN_RETRY = 2,	/* neither affirmative nor negative */
	YN_ERROR = -1	/* an error occured */
};

/*
 * Compile the regular expression expr, if that fails, compile expr2
 * instead.  Print an error message if compiling either failed.  Return
 * 0 on success, -1 on failure.
 */
static int
regex_helper(regex_t *restrict regex, const char *restrict expr, const char *restrict expr2)
{
	int compresult;
	static char errorbuf[256];

	compresult = regcomp(regex, expr, REG_NOSUB);
	if (compresult == 0)
		return 0;

	/* try to print an error message */
	regerror(compresult, regex, errorbuf, sizeof errorbuf);
	fprintf(stderr, "Error compiling regular expression %s: %s", expr, errorbuf);

	if (expr2 != NULL)
		return (regex_helper(regex, expr2, NULL));
	else
		return (-1);
}

/*
 * Get a line of text from stdin and compare it against YESEXPR and
 * NOEXPR.  If one of them matches, return YN_YES or YN_NO. If neither
 * matches, return YN_RETRY.  If an error occurs, return YN_ERROR.
 * An error message will be printed in this case, the value of errno
 * is not significant.
 */
extern int
yesno(void)
{
	static regex_t yesregex, noregex;
	static size_t inputlen = 0;
	static int haveyes = 0, haveno = 0;
	static char *inputbuf = NULL;

	/* use POSIX-locale expressions as fallback */
	if (!haveyes && regex_helper(&yesregex, nl_langinfo(YESEXPR), "^[yY]"))
		return (YN_ERROR);
	else
		haveyes = 1;

	if (!haveno && regex_helper(&noregex, nl_langinfo(NOEXPR), "^[nN]"))
		return (YN_ERROR);
	else
		haveno = 1;

	if (getline(&inputbuf, &inputlen, stdin) < 0) {
		if (ferror(stdin))
			perror("Error reading stdin");
		else
			fprintf(stderr, "End of file on stdin");

		return (YN_ERROR);
	}

	if (regexec(&yesregex, inputbuf, 0, NULL, 0) == 0)
		return (YN_YES);

	if (regexec(&noregex, inputbuf, 0, NULL, 0) == 0)
		return (YN_NO);

	return (YN_RETRY);
}