Hi there - 

I tried out your expression in a RegEx tester (successful), Nintex On-Prem (successful), and Nintex O365 (successful.) 

Here is how I had it set-up in O365 for a Single Line of Text:

Is yours different?

Cheers,

rhia

This is exactly how it looks, yet it is not working

Hmm without screen-sharing with you or being able to see specifically how it is, all I can suggest is contacting support@nintex.com and working through it with them. If it looks as the above, it should work -- it could be a super small thing missing, very hard to tell!

let us know what it ends up being!

Hi Devin,

Please make sure that you are setting the validation Required and Use Regular Expression to "Yes" and use the Regex in the Regular expression section not in the "Required - Expression" section, as shown below: 

It should work with the above settings

Regular expression working for me.

[0-9]{3}-[0-9]{2}-[0-9]{4}