Q:

c regex code not working? 

I have this code below which checks whether the user has entered a syntactically correct url.  Regex code was got from Regular expressions in C: examples? 
printf("Enter the website URL:\n");
fgets(str, 100, stdin);
if (! strcmp(str, "\n")) {
    printf("Empty URL ");
    exit(2);
}

regex_t regex;
int reti;
char msgbuf[100];

/* Compile regular expression */
reti = regcomp(&regex, "[a-zA-Z0-9\\-\\.]+\\.[a-zA-Z]{2,3}(/\\S*)?$", 0);
if (reti) {
    fprintf(stderr, "Could not compile regex\n");
    exit(3);
}

/* Execute regular expression */
reti = regexec(&regex, str, 0, NULL, 0);
if (! reti) {
    puts("Match");
} else if (reti == REG_NOMATCH) {      //This else if always executes. 
    puts("No match");
    exit(4);
} else {
    regerror(reti, &regex, msgbuf, sizeof (msgbuf));
    fprintf(stderr, "Regex match failed: %s\n", msgbuf);
    exit(5);
}

/* Free compiled regular expression if you want to use the regex_t again */
regfree(&regex);

However the regex always fails, even if the url entered is correct.  I know the regex is correct but for some reason it fails on the 'Execute regular expression' part.  Even if the user enters a syntactically correct URL the else if always executes. 
What could be the reason for the else if always executing? 

A:

Your pattern is not valid! 
Note that POSIX defines two flavors of Regex: Basic (BRE) and extended (ERE) (see Wikipedia).   Since you want to use the "extended" flavor, pass the REG_EXTENDED flag to regcomp(). 
Here are (some of?)  the problems with your pattern:
[a-zA-Z0-9\\-\\.]+\\.[a-zA-Z]{2,3}(/\\S*)

Within brackets ([]), you don't need to escape special characters.   In fact, you cannot escape them and [a-zA-Z0-9\-\.] will match backslashes, but not the hyphen, since \-\ is interpreted as the range from \ to \.  If you want to match the hyphen, place it first or last in the character list: [a-zA-Z0-9.-]
The Perl-style character class \S is not supported by POSIX.  Use [^[:space:]] instead. 
Quantifiers {} need to be written as \{\} with BRE
The + and ?  quantifiers are only supported by ERE

To summarize, replace the call to regcomp() with this one:
reti = regcomp(&regex, "[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,3}(/[^[:space:]]*)?$", REG_EXTENDED);

