$re = '/^"?(?!00[1-9])0([0-9]+)"?$/m';
$str = '666777888
0666777888
00666777888
000666777888
"666777888"
"0666777888"
"00666777888"
"000666777888"';
$subst = "$1";
$result = preg_replace($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