Description:
------------
When I updated from php 4 to php 5, one of my apps is giving this error: Warning: preg_replace_callback() [function.preg-replace-callback]: Unknown modifier '|' 

Reproduce code:
---------------
    $text = preg_replace_callback("{
            (?:\\n\\n|\\A)
            (               # $1 = the code block -- one or more lines, starting with a space/tab
              (?:
                (?:[ ]\{$md_tab_width} | \\t)  # Lines must start with a tab or a tab-width of spaces
                .*\\n+
              )+
            )
            ((?=^[ ]{0,$md_tab_width}\\S)|\\Z)  # Lookahead for non-space at line-start, or end of doc
        }xm",
        '_DoCodeBlocks_callback', $text);