C URL encoding

urlenc.c

/*
  https://gist.githubusercontent.com/sudar/3404970/raw/3f085b750aaa2ff2f49b148827a19915a4857222/url-encode.c

  gcc -o test urlenc.c -Wall
  ./test "[a b]"
  %5ba%20b%5d
*/

#include <stdio.h>

int main (int argc, char *argv[])
{
    char *hex = "0123456789abcdef", *c = argv[1];

    while(*c != '\0')
    {
        if(('a' <= *c && *c <= 'z')
        || ('A' <= *c && *c <= 'Z')
        || ('0' <= *c && *c <= '9'))
        {
          putchar(*c);
        }
        else
        {
          putchar('%');
          putchar(hex[*c >> 4]);
          putchar(hex[*c & 15]);
        }
        c++;
    }

    return 0;
}