If you're getting the same output as the input it means
the substitution is not happening. Post the s/// that you
are using and the file you are running it on. One possible
problem would be using ^User as the regex but there are spaces
before the word User in the file i.e spaces between the start of
the line which is indicated by the caret (^) symbol and the text.
Not sure what else it could be. You could put an if around the
substitution and see if it isn't happening.