$re = '/(?!&|<)&/';
$str = '<a href="http://www.testassociation.com/test.html?ab=5&cd=5&ab=c" target="_blank">Customer Association</a>&';
$subst = "&";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php