Extract URL from string

John Gruber has spent a fair amount of time perfecting the "one regex to rule them all" for link detection. Using preg_replace() as mentioned in the other answers, using the following regex should be one of the most accurate, if not the most accurate, method for detecting a link:

(?i)\b((?:[a-z][\w-]+:(?:/{1,3}|[a-z0-9%])|www\d{0,3}[.]|[a-z0-9.\-]+[.][a-z]{2,4}/)(?:[^\s()<>]+|\(([^\s()<>]+|(\([^\s()<>]+\)))*\))+(?:\(([^\s()<>]+|(\([^\s()<>]+\)))*\)|[^\s`!()\[\]{};:'".,<>?«»“”‘’]))

If you only wanted to match HTTP/HTTPS:

(?i)\b((?:https?://|www\d{0,3}[.]|[a-z0-9.\-]+[.][a-z]{2,4}/)(?:[^\s()<>]+|\(([^\s()<>]+|(\([^\s()<>]+\)))*\))+(?:\(([^\s()<>]+|(\([^\s()<>]+\)))*\)|[^\s`!()\[\]{};:'".,<>?«»“”‘’]))

There are a lot of edge cases with urls. Like url could contain brackets or not contain protocol etc. Thats why regex is not enough.

I created a PHP library that could deal with lots of edge cases: Url highlight.

You could extract urls from string or directly highlight them.
Example:

<?php

use VStelmakh\UrlHighlight\UrlHighlight;

$urlHighlight = new UrlHighlight();

// Extract urls
$urlHighlight->getUrls("This is example http://example.com.");
// return: ['http://example.com']

// Make urls as hyperlinks
$urlHighlight->highlightUrls('Hello, http://example.com.');
// return: 'Hello, <a href="http://example.com">http://example.com</a>.'

For more details see readme. For covered url cases see test.

$string = preg_replace('/https?:\/\/[^\s"<>]+/', '<a href="$0" target="_blank">$0</a>', $string);

It only matches http/https, but that's really the only protocol you want to turn into a link. If you want others, you can change it like this:

$string = preg_replace('/(https?|ssh|ftp):\/\/[^\s"]+/', '<a href="$0" target="_blank">$0</a>', $string);