How to automatically append "http://" before links before saving them to a database?

I'm developing a PHP-based web-application in which you have a form with textarea inputs that can accept lin开发者_StackOverflowks via anchor tags. But when I tested it after adding a hyperlink as follows, it pointed to a non-existent local subdirectory:

<a href="www.link.com">link</a>

I realized that this was because I had not appended http:// before the link.

there might be cases where a user might input the link just as I did above. In such cases I don't want the link to be pointing as it did above. is there any possible solution, such as automatically appending http:// before the link in case that it doesn't exist? How do I do that?

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Please consider that the anchor tags are amidst other plaintext and this is making things harder to work with.

I'd go for something like this:

if (!parse_url($url, PHP_URL_SCHEME)) {
    $url = 'http://' . $url;
}

This is an easy and stable way to check for the presence of a protocol in a URL, and allows others (e.g. ftp, https) that may be entered.

What you're talking about involves two steps, URL detection and URL normalization. First you'll have to detect all the URLs in the string being parsed and store them in a data structure for further processing, such as an array. Then you need to iterate over the array and normalize each URL in turn, before attempting to store them.

Unfortunately, both detection and normalization can be problematic, as a URL has a quite complicated structure. http://www.regexguru.com/2008/11/detecting-urls-in-a-block-of-text/ makes some suggestions, but as the page itself says, no regex URL detection is ever perfect.

There are examples of regular expressions that can detect URLs available from various sites, but in my experience none of them are completely reliable.

As for normalization, Wikipedia has an article on the subject which may be a good starting point. http://en.wikipedia.org/wiki/URL_normalization