'Processes the length definition of a netstring.
Extracts and stores in C{self._expectedPayloadSize} the number
representing the netstring size.  Removes the prefix
representing the length specification from
C{self._remainingData}.
@raise NetstringParseError: if the received netstring does not
start with a number or the number is bigger than
C{self.MAX_LENGTH}.
@param lengthMatch: A regular expression match object matching
a netstring length specification
@type lengthMatch: C{re.Match}'

def _processLength(self, lengthMatch):

    endOfNumber = lengthMatch.end(1)
    startOfData = lengthMatch.end(2)
    lengthString = self._remainingData[:endOfNumber]
    self._expectedPayloadSize = (self._extractLength(lengthString) + 1)
    self._remainingData = self._remainingData[startOfData:]
