输入一个字符串，判断其是否是C的合法标识符，请高手指教

#include"stdio.h"
void main()
{
    char a[51];
    int k,i=0,b=0;
    for (i=0;i<51;i++)
    {
        scanf("%c",&a[i]);
        if (a[i]=='\n')
        {
            k=i-1;
            break;
        }
    }
    if ((a[0]>=65 && a[0]<=90) || a[0]==95 || (a[0]>=97 && a[0]<=122))
    {
    for (i=0;i<=k;i++)
    {
        if ((a[i]>=65 && a[i]<=90) || a[i]==95 || (a[i]>=97 && a[i]<=122) || (a[i]>=48 && a[i]<=57))
         b++;
        else break;
    }
    }
    if (b==k+1) printf("yes\n");
    else printf("no\n");
}

#include <stdio.h>
#include <string.h>

static const char *Keyword[32]=
{
  "auto",    "break",   "case",     "char",
  "const",   "continue","default",  "do",
  "double",  "else",    "enum",     "extern",
  "float",   "for",     "goto",     "if",
  "int",     "long",    "register", "return",
  "short",   "signed",  "static",   "sizeof",
  "struct",  "switch",  "typedef",  "union",
  "unsigned","void",    "volatile", "while"
};


int IsKeyword(const char *s)
{

  int i;
  
  for(i=0;i<32;i++) if(!strcmp(s,Keyword[i])) return 1;
  
  return 0;
}

int IsIdentifier(const char *s)
{

   if(IsKeyword(s)) return 0;
   
   if(!isalpha(*s) && *s != '_') return 0;
   
   s++;
   
   while(*s)
   {
   
     if(!isalpha(*s) && !isdigit(*s) && *s != '_') return 0;
   
     s++;
   }
 
  return 1;
  
}


int main(void)
{
  char str[125];
  
  gets(str);
  
  if(IsIdentifier(str))  printf("it is a indentifier string");
   else printf("it is not a indentifier string");

   getch();
   return 0;
}