GREP pattern not working in JavaScript

the following pattern `\r+\z` works in indesigns grep find , replace replace blank paragraphs @ end of story doesn't seem work in scripting environment.

 

why doesn't following work:

 

app.findgreppreferences = nothingenum.nothing; app.changegreppreferences = nothingenum.nothing;  app.findgreppreferences.findwhat = '\r+\z';  app.activedocument.selection[0].changegrep(); 

\\r+\\z

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