<a href="http://example.com/">http://example.com/</a>
<a href="http://example.com/?bla=blabla">http://example.com/?bla=blabla</a>
<a href="http://example.com/">http://example.com/</a>#a
<a href="http://example.com">http://example.com</a>,
<a href="http://example.com/">http://example.com/</a>,/

Вы имеете в виду в третей строчке?

Дубль восемь :)

<?php
header('Content-Type: text/html; charset=utf-8');
$str = 'Тра та та http://example.com#name, тра ля ля';
$str1 = 'http://example.com/?bla=blabla';
$str2 = 'http://example.com/,/';
$str3 = 'http://example.com/';
$reg = '|(https?:\/\/[a-z\.\/]+([\?a-z\=\-0-9\&\#]+)?)|is';

echo preg_replace($reg, "<a href=\"\$1\">\$1</a>", $str, 1).'<br>';
echo preg_replace($reg, "<a href=\"\$1\">\$1</a>", $str1, 1).'<br>';
echo preg_replace($reg, "<a href=\"\$1\">\$1</a>", $str2, 1).'<br>';
echo preg_replace($reg, "<a href=\"\$1\">\$1</a>", $str3, 1).'<br>';