fast url checking

url.c

int is_urlschemechar(int first_flag, int ch)
{
    /*
	 * The set of valid URL schemes, as per STD66 (RFC3986) is
	 * '[A-Za-z][A-Za-z0-9+.-]*'. But use sightly looser check
	 * of '[A-Za-z0-9][A-Za-z0-9+.-]*' because earlier version
	 * of check used '[A-Za-z0-9]+' so not to break any remote
	 * helpers.
	 */
	int alphanumeric, special;
	alphanumeric = ch > 0 && isalnum(ch);
	special = ch == '+' || ch == '-' || ch == '.';
	return alphanumeric || (!first_flag && special);
}

int is_url(const char *url)
{
	/* Is "scheme" part reasonable? */
	if (!url || !is_urlschemechar(1, *url++))
		return 0;
	while (*url && *url != ':') {
		if (!is_urlschemechar(0, *url++))
			return 0;
	}
	/* We've seen "scheme"; we want colon-slash-slash */
	return (url[0] == ':' && url[1] == '/' && url[2] == '/');
}