$re = '/(\d)([() -]*)(\d)([() -]*)(\d)([() -]*)(\d)([() -]*)(\d)([() -]*)(\d)([() -]*)(\d+)/s';
$str = 'Input
PhoneNumber> (123) 556-7890 PhoneNumber>
Output PhoneNumber>(XXX) XXX-7890 PhoneNumber>
Input
PhoneNumber> 123 556 7890 PhoneNumber>
Output PhoneNumber>XXX XXX 7890 PhoneNumber>
Input
PhoneNumber> (123) 556- 7890 PhoneNumber>
Output PhoneNumber>(XXX) XXX- 7890 PhoneNumber>';
$subst = "x\2x\4x\6x\8x\10x\12\13";
$result = preg_replace($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