Booleans in C language - c

Question

#include <stdio.h>
#include <math.h>
#include <string.h>
#include <stdbool.h>
_Bool check_domain(char *domain) {
int str_count = 0, x = strlen(domain), y = 0;
if (domain[x-1] == '.' || domain[0] == '.'){
return false; //if there is a period at start or end
}
else{
while (domain[y]){
if (str_count==2){
return false; //if 2 consecutive periods
}
if (domain[y] == 46 || 65<=domain[y]<=90 || 97<=domain[y]<=122){ //if character is period or alphanumeric
if (domain[y] == '.'){
str_count += 1;
}
else{
str_count = 0;
}
}
else{ // if character is non period non alphanumeric
return false;
}
y += 1;
}
}
return true;
}
int main(void){
char domain[] = "nus#edu.sg";
printf("%d",check_domain(domain));
}
EDIT:
Thank you for your responses, have a little more clarity on the subject now.
In addition to not having a period at the end or beginning, nor two consecutive periods, there should be no non-period, non-alphanumeric character inside the string.
For reasons I can't figure out, this code fails to return false for cases wherein there are non-period, non-alphanumeric characters inside the string.

Answer 1

Start here
if (domain[y] == 46 || 65<=domain[y]<=90 || 97<=domain[y]<=122){
^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^
C can't handle statement like n <= X <= m in the way you expect.
C treats this as:
(65<=domain[y]) <= 90
^^^^^^^^^^^^^^^ ^^
bool <= number
So first C will calculate a bool result from 65<=domain[y] and then it will compare that bool to a number. The bool is converted to either 0 or 1 so when comparing with 90, the result is always true.
If you had checked the compiler warnings, you would have seen something like:
main.cpp:18:49: warning: comparison of constant '90' with boolean expression is always true [-Wbool-compare]
18 | if (domain[y] == 46 || 65<=domain[y]<=90 || 97<=domain[y]<=122){
| ^~
which tells you that something is wrong.
You need to write
if (domain[y] == 46 || (65<=domain[y] && domain[y]<=90) || (97<=domain[y] && domain[y]<=122)){
BTW:
Don't use magic numbers like 65, 90 and so on. Use the character instead. Like:
(65<=domain[y] && domain[y]<=90) --> ('A' <= domain[y] && domain[y] <= 'Z')
It makes the code much easier to understand.
Further - consider using the function isalpha(domain[y]) for such checks.
Finally - Your text (and function name) says that that you want to check for a domain name. However, your input is more like an e-mail address.