def addRule(self, field, regExpPattern):
        '''Adds new rule for checking whether a value of the field matches given regular 
           expression regExpPattern;
        
           Parameters
           ----------
           field: str
                keyword, e.g. 'partofspeech', 'root', 'text' etc
           regExpPattern: str
                a regular expression that the value of the field must match (using method 
                re.match( regExpPattern, token[field]) ).
        '''
        compiled = re.compile( regExpPattern )
        if field in self.analysisFields:
            if self.analysisRules == None:
                self.analysisRules = dict()
            self.analysisRules[field] = compiled
        else:
            if self.otherRules == None:
                self.otherRules = dict()
            self.otherRules[field] = compiled