Using regexp

Hello everybody

In a textfield, I have got several lines with a carriage return. I would like to delete only the last carriage return of the last line. How can I do that with a regexp. In InDesign, the regexp is : search (?=\r)\r and replace with nothing.

But in LiveCycle

In the exit event:

var str = this.rawValue;

this.rawValue = str.replace(/ what here? /,"");

Many thanks