Is there a way to check if a string can be a float in C?

Question

Checking if it can be an int is easy enough -- just check that every digit is between '0' and '9'. But a float is harder. I found this, but none of the answers really work. Consider this code snippet, based on the top (accepted) answer:

float f;
int ret = sscanf("5.23.fkdj", "%f", &f);
printf("%d", ret);

1 will be printed.

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Another answer suggested using strpbrk, to check if certain illegal characters are present, but that wouldn't work either because 5fin7 wouldn't be legal, but inf would.

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Yet another answer suggested checking the output of strtod. But consider this:

char *number = "5.53 garbanzo beans"
char *foo;

strtod(number, &foo);

printf("%d", isspace(*foo) || *foo == '\0'));

It'll print 1. But I don't want to remove the isspace call entirely, because " 5.53 " should be a valid number.

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Is there a good, elegant, idiomatic way to do what I'm trying to do?

Answer 1

The first answer should work if you combine it with %n, which is the number of characters read:

int len;
float ignore;
char *str = "5.23.fkdj";
int ret = sscanf(str, "%f %n", &ignore, &len);
printf("%d", ret==1 && !str[len]);

!str[len] expression will be false if the string contains characters not included in the float. Also note space after %f to address trailing spaces.

Demo

Answer 2

You could check if - after having read a value using strtod - the remainder consists solely of white spaces. Function strspn can help here, and you can even define "your personal set of white spaces" to consider:

int main() {

    char *number = "5.53 garbanzo beans";
    char *foo;

    double d = strtod(number, &foo);
    if (foo == number) {
        printf("invalid number.");

    }
    else if (foo[strspn(foo, " \t\r\n")] != '\0') {
        printf("invalid (non-white-space) trailing characters.");
    }
    else {
        printf("valid number: %lf", d);
    }
}