http://stackoverflow.com/q/8094479

calc.c

// gcc -std=c99 -Wall -pedantic -Wextra *.c -lreadline && ./a.out
#include <limits.h> // INT_MAX
#include <stdio.h>
#include <stdlib.h> // strtol

#include <readline/readline.h>

// space = \s+
// skip one or more ' ' characters
// return a pointer to the first non-' ' character or NULL on error
// NOTE: there must be at least one space
// NOTE: '\t', '\f', '\v' are *NOT* considered to be a space character here
static const char*
space(const char* pc) {
  if (! (pc && *pc == ' ')) return NULL; // error

  ++pc;
  while(*pc == ' ') ++pc;
  return pc;
}

// operator = [+-]
// parse operator and store it in *presult (either '+' or '-')
// return a pointer to the first character after the operator or NULL on error
static const char*
operator(const char* pc, int *presult) {
  if (! (pc && presult)) return NULL; // error

  if (*pc == '+' || *pc == '-') {
    *presult = *pc;
    return ++pc;
  }
  return NULL;
}

// number = -?\d+
// parse the string as an integer and store it in *presult
// return a pointer to the first character after the number or NULL on error
static const char*
number(const char* pc, int* presult) {
  if (! (pc && presult)) return NULL; // error

  int sign = 1;
  if (*pc == '-') { //NOTE: strtol() is too permissive, take sign by hand
    ++pc;
    sign = -1;
  }
  char *end = NULL;
  long int result = strtol(pc, &end, 10);
  if (end == pc || result < 0 || result > INT_MAX) return NULL;

  *presult = sign*result;
  return end; // ok
}

// expr = number (\s+ [+-] \s+ number)*
// parse and evaluate the expression; store the result in *presult
// return a pointer to '\0' or NULL on error
static const char*
expr(const char* pc, int* presult) {
  if (! (pc && presult)) return NULL;  // error

  int a, op, b;
  if (! (pc = number(pc, &a))) return NULL;

  while (*pc != '\0') {
    if (! (pc = space(pc)))		return NULL;
    if (! (pc = operator(pc, &op)))	return NULL;
    if (! (pc = space(pc)))		return NULL;
    if (! (pc = number(pc, &b)))	return NULL;

    // evaluate
    if (op == '+') a += b;      //NOTE: ignore overflow
    else if (op == '-') a -= b;
    else return NULL; // can't happen
  }
  *presult = a;
  return pc; // ok
}

int main() {
  char* line = NULL;
  while ((line = readline("calc> ")) && *line != '\0') {
    int result = 0;
    if (! expr(line, &result)) exit(EXIT_FAILURE);
    printf("%d\n", result);
    free(line);
  }
  exit(EXIT_SUCCESS);
}