If the dot is the first dot in the file, this will remove everything after it, but retains the dot.
If you want to also remove the dot, leave Replace with empty.
When using a regular expression, be sure to mark the option Find-Pattern is a regular expression as "Yes".
To test out expressions, this is a nice online tool: https://www.regextester.com/ but you can google for others. Or just test within Galaxy on a representative subset of your data, help is on the tool form.
Thanks, Jen, Galaxy team