validation of nd number added

Closes #148 (closed)

smash/web/models/subject.py

 # coding=utf-8
+from django.core.validators import RegexValidator
 from django.db import models
 
 from constants import BOOL_CHOICES, SEX_CHOICES, SUBJECT_TYPE_CHOICES, COUNTRY_OTHER_ID
@@ -124,7 +125,9 @@ class Subject(models.Model):
                                         )
     nd_number = models.CharField(max_length=25,
                                  blank=True,
-                                 verbose_name='ND number'
+                                 verbose_name='ND number',
+                                 validators=[
+                                     RegexValidator('^(ND[0-9]{4}|)$', message="ND number should look as follows: NDxxxx")]
                                  )
     mpower_id = models.CharField(max_length=20,
                                  blank=True,