$re = '/(?:([a-z])([A-Z]))|(?:((?!^)[A-Z])([a-z]))/m';
$str = 'AbcItem
ABCItem
TestAbcItem
testABCItem
TestItemAbc
TestItemABC
abcItem
testABCItem
';
$subst = "$1_$3$2$4";
$result = preg_replace($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