@KellyAlexander @BenVan_Kesteren All,
Through several community posts and a lot of testing, I've strung together how to restrict multiple, but not all, special characters in a text field. Specifically, to restrict \ /, use
not(regex(.,'[|\x5C/]'))
I restricted ' \ & @ ; " / ? + # $ using
not(regex(.,'[|\x27|\x5C&@;"/?+#$]'))
For a single quote ' and backslash \, I found to use |\x%% with %% corresponding to the URL encoding code for the special character.
To restrict use of all special characters and allow letters and numbers only, use
regex(.,'^[A-Za-z0-9]*$')