A couple easy questions for a Nintex Pro...

Marian,

Thanks for the reply.  My field is a Single Line Textbox and I'm not sure how to get the "Formula" dialog that you have there (you're using a calculated value and the Control Settings are different).

I'm not having a lot of luck getting this to work, but also, my main goal was to disallow forbidden characters, not replace them.  I want to drive the behavior in my users that they shouldn't use forbidden characters in the item titles.

First off, I need to disallow forbidden characters from the item title field, with a message that says something like "Your title has forbidden characters...." if someone tries to enter them. 

Alright. This is easily doable. If you want to just replace all of the characters that are invalid with valid ones, that's also doable, but it would remove any reason to show a warning, and doesn't encourage your users to get it right... So...

Let's inform the user that they have made a mistake (and force them to change their filename for us)!

Here's a typical Single Line Text control that has been linked to the Title column.

To make our message, we need to create a Validation Rule

The rule setup will look like this: 

Here is the code, explicitly (you can just copy / paste that right in): 

{Control:Self}.match(/[^A-Za-z0-9-_ ]/gi) !== null‍

The Regex (/[^A-Za-z0-9-_ ]/g) is trying to match any character that ISN'T A through Z (uppercase), a through z (lowercase), 0 through 9, a dash -, an underscore _, or a space. An attempt to use any other character will result in an error being thrown. 

If there are characters that are allowed which have been missed by the above regex, you can add them at your discretion (or remove them if I have included an invalid character, like, space). 

As for your other topic about the hiding of select options, I would create a different forum topic to discuss that, so that we don't muddy the waters on this one. 

Does this help get you going in the right direction?