rename file with no extension regex incorrect

[Scarecrow1965]

I have tried numerous times to use the prefix listed in this line within the document:
https://learn.microsoft.com/en-ca/windows/powertoys/powerrename

When using the variables, "Match all occurrences" must be selected.
(^\w+.$)\|(^\w+$) | $2.txt | Appends ".txt" extension to existing file name only if it does not have an extension

Your program will not accept this regex.

Using a regex editor I was able to find the correct expression:
^([^.]+)$

Therefore the correct lines of text should read:
When using the variables, "Use Regular Expressions" and "Match all occurrences" must be selected.
^([^.]+)$ | $1.txt | Appends ".txt" extension to existing file name only if it does not have an extension