How does it work just ain't kicking in

[Frederick]

Question: How does the following code work, I know how to manipulate a little, but can expand on it if I know more of how it works.
The below code came from php.net

$text = ereg_replace("[[:alpha:]]+://[^<>[:space:]]+[[:alnum:]/]",
                     "<a href=\"\\0\">\\0</a>", $text);

How does the \0 come into play.
Also, how to use preg_replace for using BB Code items in content. This has the same \1 item in it.

$value = preg_replace("|\[url](.+)\[/url]|i","<a href=\"http://\\1\" target=\"_blank\">\\1</a>",$str);

[kburger]

Regular expressions is a complicated subject. You might want to start here

[Shrike]

\0, \1 and so on refer to the matches made in the matching portion of the regexp. They can also be referred to as $1, $2.

$in = "ab";
$out = preg_replace("/(\\w)(\\w)/s", "$2$1", $in);
echo $out; // echoes "ba"