/(                      # Capture group 1
     \(                  # Opening paren
         (?:             # Non-capture group
             [^()]++     # Match non-brackets, no backtracking
             |
             (?1)        # Recurse to group 1. (?R) works, too.
         )*              # Zero or more times
     \)                  # Closing paren
 
     | [^+()]            # Anything outside of parens not a + or paren
 
  )+                     # Not *, assuming you only want addition, not
                         # +positive integers at beginning of string
  
  (\+ .*)                # Plus sign, and remainder of string
 /x;