Regex for a simple text addition might be an overkill, but if you do the 'replace' box should be filled with what already exist in the title, as a regex and the 'with' box with your content. Example, if you want to replace all current text in title with 'toto', then write "^. *$" in the 'replace' box and "toto" in the other one. Use only "^" to add toto before existing content in the title, or "$" to add toto at the end.
A better way would be to use the String builder module. however you need to change the title for all items, so put it in a Loop module. It's easier to use.