simple expression parser

parser.c

/*
 * simple expression parser
 * example: 1+2-3  -> 12+3-
 */
#include <stdio.h>
#include <ctype.h>
#include <stdlib.h>
#include <stdbool.h>

int lookahead;
void parser ()
{
  lookahead = getchar();
}

void expr ()
{
	term();
	while (true)
	{
		if (lookahead == '+')
		{
			match('+');
			term();
			putchar('+');
		}
		else if (lookahead == '-')
		{
			match('-');
			term();
			putchar('-');
		}
		else
			return;
	}
}

void term()
{
	if (isdigit((char)lookahead))
	{
		putchar((char)lookahead);
		match(lookahead);
	}
	else
	{
		fprintf(stderr,"syntax error");
		exit(EXIT_FAILURE);
	}
}

void match(int t)
{
	if (lookahead == t)
		lookahead = getchar();
	else
	{
		fprintf(stderr,"syntax error");
		exit(EXIT_FAILURE);
	}
}

int main()
{
	parser();
	expr();
	putchar('\n');

	return 0;
}