Description:
------------
mb_ereg_replace() works incorrectly if 2nd parameter is multibyte and contains substings \\n (n - digit). It returns substing \n itself instead of corresponding parenthesized substring.

If 2nd parameter not multibyte - \\n correctly replaced with corresponding parenthesized substrings.

If 2nd parameter multibyte, but not contains \\n substrings - replacement works correctly.

Reproduce code:
---------------
<?php
                                      
function a2u ( $a ) {
    return mb_convert_encoding ( $a, "UTF-16LE", "ASCII" );
    }

function u2a ( $a ) {
    return mb_convert_encoding ( $a, "ASCII", "UTF-16LE" );
    }
                                      
mb_internal_encoding ( "UTF-16LE" );
mb_regex_encoding ( "UTF-16LE" );
                                      
$str = "abc..def";
$pat = "([^\.])\.\.([^\.])";
$rep = "\\1...\\2";
                                                                
$res = mb_ereg_replace ( a2u ( $pat ), a2u ( $rep ), a2u ( $str ) );
echo "{" . $str . "}{" . u2a ( $res ) . "}\n";

?>

Expected result:
----------------
{abc..def}{abc...def}


Actual result:
--------------
{abc..def}{ab\1...\2ef}