(?:
    (?{ local $d=0 }) # set depth to 0
    (                 # start capture
      (?:                  
        \(            # opening paren
        (?{$d++})     # increment the depth
        |  
        \)            # or closing paren
        (?{$d--})     # decrement the depth
        (?(?{$d==0})  # if we're back to the start
          (*ACCEPT)   # we're done
        )
        |
        (?>[^()]*)    # or there's just some normal text
      )*
      (?(?{$d!=0})    # done. did we reach a matching closing paren
        (?!)          # nope, failed
      )
    )                 # end capture
  )