[squidsk]

Your playing around with regular expressions so you need to escape every non-standard character otherwise they all have particular meanings for the regular expression engine other than their plain text characters.

So your search string would need to be something like:

'\<img src=\"\$post\[avatarurl\]\" \$post\[avwidth\] \$post\[avheight\] alt=\"\<phrase 1=\"\$post\[username\]\"\>\$vbphrase\[xs_avatar\]\<\/phrase\>\" border=\"0\" \/\>'

The easiest way to test is to use an online preg_replace tester to see if you patterns work or not.