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JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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That is possible by setting a validation to your text field. You can set it as 'numbers' then apply input mask so you can format the numbers that will be inputted into your text fields.
You can set your input mask like this:
But I don't want the user to know how many digits needs to be entered.
Unfortunately this is not possible without using your forms source code and embedding it to a web page. Here is a discussion on how to accomplish this if you are willing to do so:
Essentially you would need a script such as this one in your page to run the validation:
Then you would substitute "Input_5" with the field ID in your form you wish to validate.
Please let us know if you have any further questions and we will be happy to help.
Thank you for using JotForm!