Replaces non-anchor wrapped urls with anchors, also filters attributes on current anchors. This is a Gist so that if you have an issue you can

LinkLocator.php

<?php

$string = 'Lorem ipsum dolor sit amet, consectetur http://google.com</a> adipiscing elit. http://google.com/ Pellentesque vel lectus nec arcu malesuada eleifend eget non lectus. Nullam id tortor velit. Pellentesque ac nunc dui. Nunc dapibus, metus ac ullamcorper <a href="http://google.com" onclick="fff d">luctus</a>, arcu dolor pulvinar massa, ut gravida justo nulla a risus. Integer sit amet urna ut mauris scelerisque sollicitudin. Nulla fringilla ligula nec mi vehicula in hendrerit libero volutpat. <a href="http://google.com">http://google.com</a>';


class LinkLocator {
	
	//If you change this regex, change the duplicate bit in the main regex. I should really be doing this progmatically
	protected static $attrRegex = '(?:\S+?=(?:(?:\'.*?\')|(?:".*?")\s*))';
	
	protected static $replaceRegex = '/(?<anchor><a(?:\s+(?<attr>(?:\S+?=(?:(?:\'.*?\')|(?:".*?")\s*))+))?>(?<text>.*?)<\/a\s*>)|(?<!>)(?<url>(?<proto>https?:\/{2})(?<domain>[a-zA-Z0-9\-.]+\.[a-zA-Z]{2,3})(?<path>\/\S*)?)/i';
	
	protected static $filterAttributes = true;
	protected static $allowedAttributes = array('href');
	
	public static function parseString ($parseString) {
		
		//use an anonymous function if you desire, but it will only work in PHP > 5.3
		return preg_replace_callback(self::$replaceRegex, array('self', 'linkCallback'), $parseString);
	
	}

	protected static function linkCallback($matches) {
		if (strlen($matches['anchor']) > 0) {
			
			if (self::$filterAttributes) {
				$attributeString = '';
				
				if (strlen($matches['attr']) > 0) {
					preg_match_all('/' . self::$attrRegex . '/i', $matches['attr'], $attributes);
					
					foreach ($attributes[0] as $attribute) {
						$attibuteSplit = explode('=', $attribute);
						
						if (in_array($attibuteSplit[0], self::$allowedAttributes)) {
							$attributeString .= ' ' . $attribute;
						}
					}
				}
			} else {
				$attributeString = ' ' . $matches['attr'];
			}
			
			return '<a' . $attributeString . '>' . $matches['text'] . '</a>';
			
		} else {
		
			$url =  $matches['proto'] . $matches['domain'] . $matches['path'];
			return '<a href="' . $url . '">' . $url . '</a>';
			
		}
	
	}
	
}

echo LinkLocator::parseString($string);