RegexValidationWithMessage.swift

public func RegexValidationWithMessage(_ message:String, regex:String, allowingNull:Bool = true) -> Validation<String>? {
    
    if let regex = try? NSRegularExpression(pattern: regex, options: .CaseInsensitive) {
        return Validation<String>(message: message, validation: { (value) -> Bool in
            let nullValidation = NonEmptyStringValidation("")
            
            guard let stringValue = value else {
                return false
            }
            
            if !allowingNull && nullValidation.validate(value: stringValue) != .Valid {
                return false
            }
            
            if allowingNull && stringValue.isEmpty {
                return true
            }
        
            return regex.matchesInString(stringValue, options: NSMatchingOptions.ReportProgress, range: NSMakeRange(0, stringValue.characters.count)).count > 0
            
        })
    } else {
        return nil
    }
}