You can restrict input from your visitor using an input mask. For example, you may want visitors to enter phone numbers only in a specific format.
Go to Marketing Activities.
Select your form and click Edit Form.
Select your field and make sure the Field Type is set to Text.
Click the Mask Input link.
Enter your input mask and click Save.
Click Approve and Close.
Check it out! Now you're asking the visitor to enter numbers in a specific format.
Pretty cool, huh?