<?php
$str = '<a href="def.html"></a><a href="def2.html"></a>';
$prefix = "http://www.abc.com/";
echo htmlSpecialChars(preg_replace("/(href=)([\'\"])(?!http:\/\/)(.+)\\2/U", "\\1\\2{$prefix}\\3\\2", $str));
?>
$str = '<a href="def.html"></a><a href="def2.html"></a>';
$prefix = "http://www.abc.com/";
echo htmlSpecialChars(preg_replace("/(href=)([\'\"])(?!http:\/\/)(.+)\\2/U", "\\1\\2{$prefix}\\3\\2", $str));
?>
preg_replace("/(href=)([\'\"])(?!http:\/\/)([^\\2]+)/U", "\\1\\2{$prefix}\\3", $str);