<a href="url">url</a>

_^(?:(?:https?|ftp)://)(?:S+(?::S*)?@)?(?:(?!10(?:.d{1,3}){3})(?!127(?:.d{1,3}){3})(?!169.254(?:.d{1,3}){2})(?!192.168(?:.d{1,3}){2})(?!172.(?:1[6-9]|2d|3[0-1])(?:.d{1,3}){2})(?:[1-9]d?|1dd|2[01]d|22[0-3])(?:.(?:1?d{1,2}|2[0-4]d|25[0-5])){2}(?:.(?:[1-9]d?|1dd|2[0-4]d|25[0-4]))|(?:(?:[a-zx{00a1}-x{ffff}0-9]+-?)*[a-zx{00a1}-x{ffff}0-9]+)(?:.(?:[a-zx{00a1}-x{ffff}0-9]+-?)*[a-zx{00a1}-x{ffff}0-9]+)*(?:.(?:[a-zx{00a1}-x{ffff}]{2,})))(?::d{2,5})?(?:/[^s]*)?$_iuS

/_(?:(?:https?|ftp)://)(?:S+(?::S*)?@)?(?:(?!10(?:.d{1,3}){3})(?!127(?:.d{1,3}){3})(?!169.254(?:.d{1,3}){2})(?!192.168(?:.d{1,3}){2})(?!172.(?:1[6-9]|2d|3[0-1])(?:.d{1,3}){2})(?:[1-9]d?|1dd|2[01]d|22[0-3])(?:.(?:1?d{1,2}|2[0-4]d|25[0-5])){2}(?:.(?:[1-9]d?|1dd|2[0-4]d|25[0-4]))|(?:(?:[a-zx{00a1}-x{ffff}0-9]+-?)*[a-zx{00a1}-x{ffff}0-9]+)(?:.(?:[a-zx{00a1}-x{ffff}0-9]+-?)*[a-zx{00a1}-x{ffff}0-9]+)*(?:.(?:[a-zx{00a1}-x{ffff}]{2,})))(?::d{2,5})?(?:/[^s]*)?_iuS/g

preg_replace($regex, '<a href="$0">$0</a>', $examplestring);

If this modifier is set, letters in the pattern match both upper and lower
case letters.

This modifier inverts the "greediness" of the quantifiers so that they are
not greedy by default, but become greedy if followed by ?. It is not compatible
with Perl. It can also be set by a (?U) modifier setting within the pattern
or by a question mark behind a quantifier (e.g. .*?).

When a pattern is going to be used several times, it is worth spending more
time analyzing it in order to speed up the time taken for matching. If this
modifier is set, then this extra analysis is performed. At present, studying
a pattern is useful only for non-anchored patterns that do not have a single
fixed starting character.