Question

I’ve been searching for a regex to replace plain text url’s in a string (the string can contain more than 1 url), by:

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

and I found this:
http://mathiasbynens.be/demo/url-regex

I would like to use the diegoperini’s regex (which according to the tests is the best):

_^(?:(?: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]|2\d|3[0-1])(?:\.\d{1,3}){2})(?:[1-9]\d?|1\d\d|2[01]\d|22[0-3])(?:\.(?:1?\d{1,2}|2[0-4]\d|25[0-5])){2}(?:\.(?:[1-9]\d?|1\d\d|2[0-4]\d|25[0-4]))|(?:(?:[a-z\x{00a1}-\x{ffff}0-9]+-?)*[a-z\x{00a1}-\x{ffff}0-9]+)(?:\.(?:[a-z\x{00a1}-\x{ffff}0-9]+-?)*[a-z\x{00a1}-\x{ffff}0-9]+)*(?:\.(?:[a-z\x{00a1}-\x{ffff}]{2,})))(?::\d{2,5})?(?:/[^\s]*)?$_iuS

But I want o make it global to replace all the url’s in a string.
When I use this:

/_(?:(?: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]|2\d|3[0-1])(?:\.\d{1,3}){2})(?:[1-9]\d?|1\d\d|2[01]\d|22[0-3])(?:\.(?:1?\d{1,2}|2[0-4]\d|25[0-5])){2}(?:\.(?:[1-9]\d?|1\d\d|2[0-4]\d|25[0-4]))|(?:(?:[a-z\x{00a1}-\x{ffff}0-9]+-?)*[a-z\x{00a1}-\x{ffff}0-9]+)(?:\.(?:[a-z\x{00a1}-\x{ffff}0-9]+-?)*[a-z\x{00a1}-\x{ffff}0-9]+)*(?:\.(?:[a-z\x{00a1}-\x{ffff}]{2,})))(?::\d{2,5})?(?:/[^\s]*)?_iuS/g

It does not work, how do I make this regex global and what does the underscore at the beginning and the “_iuS”, at the end, means?

I would like to use it with php so I am using:

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

Answer 1

The underscores are the regex delimiters, the i, u and S are pattern modifiers :

i (PCRE_CASELESS)

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

U (PCRE_UNGREEDY)

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. .*?).

S

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.

For more informations see http://www.php.net/manual/en/reference.pcre.pattern.modifiers.php

When you added the / … /g , you added another regex delimiter plus the modifier g wich does not exists in PCRE, that’s why it did not work.