title: Emacs: how to replace a string using a regular expression?
body: <p>suppose i have the following in a text file (or dired buffer) open in emacs:
  file_01.txt
  file_02.txt
  ...
  file_99.txt</p>
  
  <p>I want to query-replace or replace the files to
  01_file.txt, etc.</p>
  
  <p>I want to use query-replace-regexp or replace-regexp, but don't know what to put in.  The search part i put in "file_..", but the ".." are read as periods in the replacement string.  I'm beginning to learn regexp and don't know how to do this.  Please help, thanks.</p>