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C program to test whether a given identifier is valid or not

 LOGIC: Read the given input string.Check the initial character of the string is numerical or any special character except ‘_’ then print it is not a valid identifier.Otherwise print it as valid identifier if remaining characters of string doesn’t contains any special characters except ‘_’. 
PROGRAM:
#include<stdio.h>
#include<conio.h>
#include<ctype.h>
void main()
{
char a[10];
int flag, i=1;
clrscr();
printf("\n Enter an identifier:");
gets(a);
if(isalpha(a[0]))
flag=1;
else
printf("\n Not a valid identifier");
while(a[i]!='\0')
{
if(!isdigit(a[i])&&!isalpha(a[i]))
{
flag=0;
break;
}
i++;
}
if(flag==1)
printf("\n Valid identifier");
getch();
}
 OUTPUT:
Enter an identifier: first
Valid identifier

Enter an identifier:1aqw
Not a valid identifier

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