Is complex field validation possible with WebForm?
Try using the WebForm Validation module.


  The following validation rules are currently included:
  
  
  Numeric values (optionally specify min and / or max value)
  Minimum length
  Maximum length
  Minimum number of words
  Maximum number of words
  Equal values on multiple fields
  Unique values on multiple fields
  Specific value
  Require at least one of two fields
  Require at least one of several fields
  Minimum number of selections required
  Maximum number of selections allowed
  Exact number of selections required
  Plain text (disallow tags)
  Regular expression
  Must be empty (Anti-Spam: Hide with CSS)  
  Words blacklist  
  Must match a username