How to Validate and Restrict Phone Number Input in Ninja Forms

Many users of the 'Ninja Forms – The Contact Form Builder That Grows With You' plugin need to ensure that phone number fields only accept digits and adhere to a specific length. This is a common requirement for creating professional, user-friendly forms that collect accurate data. This guide covers the most effective methods to implement this validation.

Understanding the Core Phone Number Field

By default, the Phone number field in Ninja Forms is designed to primarily accept numeric input. However, based on user reports, it may not always strictly prevent non-numeric characters in all scenarios. For basic numeric-only validation, this field is the recommended starting point.

Solution 1: Using a Custom Input Mask (For Specific Lengths)

The most robust way to enforce a specific number of digits (e.g., a 10-digit US number or a 9-digit international number) is to use a custom input mask. This feature forces the field's format, preventing users from entering more or fewer characters than specified.

How to set it up:

1. Edit your form and click on the phone number field to open its settings.
2. Navigate to the Restrictions tab.
3. Locate the Input Mask option. If you cannot see it, you may need to enable Developer Mode.
   • Go to Ninja Forms > Settings > Advanced tab.
   • Check the box for Dev Mode and save changes.
   • Return to your field's settings; the option should now be visible.
4. In the Input Mask field, enter a series of '9's, with each '9' representing a single digit. For a 10-digit number, you would enter: 9999999999.
5. Save the form and test. The field will now only accept the exact number of digits defined by the mask.

Solution 2: Using a Number Field (For Strict Numeric Input)

For cases where you absolutely must prevent any non-numeric character, using a standard Number field instead of a Phone field is a viable alternative. The Number field is designed by HTML standards to only accept numbers.

Note: Some users have reported a rare bug where extremely large numbers in a Number field can trigger an incorrect "Please increment by 1" validation error. This is not typical for standard phone number lengths but is something to be aware of. If you encounter this with a normal phone number, it may indicate a unique site-specific conflict.

Addressing the "Increment by 1" Error

As mentioned in several community threads, a small number of users encounter a "Please increment by 1" error in Number fields, even with reasonably sized numbers like a UK mobile number. This appears to be an uncommon bug that can be caused by conflicts with other plugins or a site's theme. If you experience this issue with a Number field, the recommended course of action is to use the Phone field with an input mask instead, as it is generally unaffected by this specific error.

Why Some Options Are Hidden in Developer Mode

The 'Ninja Forms – The Contact Form Builder That Grows With You' team has stated that advanced options like the input mask are placed behind the Developer Mode setting to reduce clutter and simplify the form building experience for the majority of users. This is a design decision aimed at improving usability, though it can make certain powerful features less discoverable.

Summary and Best Practices

• For most use cases, use the Phone field with a custom Input Mask (enabled via Developer Mode) to control the number of digits.
• If you require the strictest numeric-only input and a specific digit count is less critical, consider using a Number field.
• If you encounter the "Please increment by 1" bug on a Number field, switching to a masked Phone field is the most straightforward workaround.

These methods should help you effectively validate phone number input and ensure your forms collect clean, usable data from your visitors.