CODEWITHSUNDEEP
cstringsubstringc-stringsstrstr
NobleUplift
NobleUplift

Reputation: 6034

Will strstr return the same pointer as the first string if the second string is at the beginning?

So basically, I need to find out if a string begins with [URL] and ends with [/URL].

Right now I'm doing:

const char *urlStart;
// ...
urlStart = strstr(message, "[URL]");
if (urlStart == NULL) {
    urlStart = strstr(message, "[url]");
}

if (urlStart == NULL) {
    return NULL;
}

According to cplusplus.com, "a pointer to the first occurrence in str1 of the entire sequence of characters specified in str2".

Does that mean I can just do this?

/*
 * If the pointer to message is the same as urlStart,
 * message begins with urlStart
 */
if (message != urlStart) {
    return NULL;
}

// URL starts 5 characters after [URL]
urlStart += 5;

Initial testing seems to indicate that this doesn't work.

The complete function is located here.

Upvotes: 1

Views: 925

Answers (1)

user3386109
user3386109

Reputation: 34839

Yes, the check if (message != urlStart) will work as expected, assuming that message begins with either [URL] or [url]. However if, for example, message starts with [Url], then strstr won't find the string because of case mismatch.

Given that you require the string to be at a known location in message, the strstr function really doesn't do much for you. It's simpler just to check the first 5 characters of message, like this

char *start = "[URL]";
for ( int i = 0; i < 5; i++ )
    if ( toupper(message[i]) != start[i] )
        return NULL;

And you can check the [/URL] at the end like this

length = strlen(message);
if ( length < 12 )
    return NULL;

char *end = "[/URL]";
for ( int i = 0; i < 6; i++ )
    if ( toupper(message[length-6+i]) != end[i] )
        return NULL;

You could also use the non-case-sensitive-length-limited string compares, but note that those aren't portable. I believe it's strnicmp on Windows, and strncasecmp on unix clones.

Upvotes: 3

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