Form Validation in OutSystems: Built-in and Custom Patterns

Open your screen in the Screen Editor. In the Toolbox (left panel), search for 'Form' and drag it onto the canvas. Inside the Form, drag three Input widgets from the Toolbox. Select each Input → in the Properties Panel (right side), set the Variable property: first Input → create a new Local Variable 'FullName' (Text), second → 'Email' (Email type), third → 'BirthDate' (Date). The Email data type gives you built-in format validation for free. The Date type enforces date format automatically.

Select the FullName Input widget. In the Properties Panel, find the Mandatory property and set it to True. Repeat for the Email Input. Leave BirthDate as non-mandatory for now. When a user submits without filling a Mandatory field, OutSystems automatically sets that input's Valid property to False and displays a default 'Required field.' message below the input. No Client Action logic is needed for this basic check.

Drag a Button widget from the Toolbox into the Form. In the Properties Panel, set the Button's Label to 'Register'. Find the Validation property (under the Events section in the Properties Panel) and set it to 'Client & Server'. This tells OutSystems to run client-side validation first (Mandatory checks, type checks) before calling any server logic. Next, right-click the Button → 'Add OnClick Client Action'. Service Studio creates a new Client Action and opens it in the Action Flow Editor.

In the Action Flow Editor, you should see Start → End. Add your custom checks before calling any server logic. Action flow: Start → If (Length(Trim(FullName)) < 2) → [True] Assign (set FullName.Valid = False, FullName.ValidationMessage = 'Name must be at least 2 characters') → [False path continues] → If (Form.Valid) → [True] call RegisterUser Server Action → [False] End. To add the Assign node: drag Assign from the Toolbox into the flow. In the Assign Properties, click '+' to add assignments: Variable = FullNameInput.Valid, Value = False. Add a second row: Variable = FullNameInput.ValidationMessage, Value = 'Name must be at least 2 characters'. The key pattern is: set individual input Valid/ValidationMessage first, then gate on Form.Valid — OutSystems automatically makes Form.Valid false when any child input is invalid.

1/* Expression for the If condition */
2Length(Trim(FullName)) < 2
3
4/* Assign node values */
5FullNameInput.Valid = False
6FullNameInput.ValidationMessage = "Name must be at least 2 characters"

After the name length check, add another If node in the action flow. Condition: use the built-in OutSystems expression to check the email format. OutSystems does not have a built-in Regex() function exposed in expressions, but you can use the EmailValidator pattern from Forge, or rely on the Email data type which enforces basic format. For a custom rule, use a JavaScript node to call a regex. Alternatively, the cleanest OutSystems-native approach is: set the Email input's data type to 'Email' (which enforces RFC-basic format automatically) and add an additional custom If check for your business rule, e.g. corporate email domain only: If (not (Index(Email, "@company.com", 0, False, True) > 0)) → Assign EmailInput.Valid = False, EmailInput.ValidationMessage = 'Use your company email address'

1/* If condition for corporate domain check */
2not (Index(Email, "@company.com", 0, False, True) > 0)
3
4/* Assign node */
5EmailInput.Valid = False
6EmailInput.ValidationMessage = "Use your company email address"

After the If (Form.Valid) true branch, add a Message node (drag from Toolbox) to give the user confirmation, then optionally navigate away. Action flow for the True branch: → call RegisterUser Server Action → Message ('Registration successful!', MessageType Entities.MessageType.Success) → Destination node to navigate to a confirmation screen. If you prefer a toast notification: use the built-in feedback from the screen's OnNotification handler or use $public.FeedbackMessage.showFeedbackMessage() via a JavaScript node.