Hi,
while there is no direct way to do external validation you can since V5 use a method that will check if a respondent answer set has invalid answers in it or not.
Its in the SurveyAnswer class and is called
public List<InvalidSurveyQuestion> ValidateRespondentAnswers(int surveyId, int pageNumber,
bool enableAnswerDefaults, VoterAnswersData respondentAnswers, bool applyQuestionSkipLogic,
string languageCode, string userId, string[] userGroups, bool breakOnFirstError)
eg:
it will check the VoterAnswersData that you pass in and see if all conditions (mandatory, regex etc..) have been met and return all invalid questions (if any). You can set applyQuestionSkipLogic, breakOnFirstError and enableAnswerDefaults to false and leave language code, user id, user groups to null. Set Page to -1 if you want to check all pages in the survey and pass as respondentanswers the respondent answer dataset that you whish to check