title: ag Silver Searcher: rules for lookahead/lookbehind patterns?
body: <p>I'm using <code>ag</code> in Vim with good results for string/file search.</p>
  
  <p>However, there seems to be not much documentation how patterns are constructed for ag.</p>
  
  <p>I'm trying to use ag instead of vimgrep in an example from the Practical Vim book.</p>
  
  <p>I want to find every occurrence of "Pragmatic Vim" in all files in a directory recursively, and substitute this string with "Practical Vim". There is also "Pragmatic Bookshelf" in some files, and that string must remain.</p>
  
  <p>the book suggests this approach:</p>
  
  <pre><code>/Pragmatic\ze Vim
  :vimgrep /&lt;C-r&gt;// **/*.txt
  </code></pre>
  
  <p>And after that step, populate quickfix list with <code>:Qargs</code> plugin command, and, finally <code>:argdo %s//Practical/g</code></p>
  
  <p>Now, how do I specify <code>/Pragmatic\ze</code>pattern using <code>ag</code>?</p>
  
  <p>Is <code>ag</code> at all designed for what I'm trying to do here?</p>