title: using captured groups in str_replace / stri_replace - stringi vs stringr
body: <p>Most <code>stringr</code> functions are just wrappers around corresponding <code>stringi</code> functions. <code>str_replace_all</code> is one of those. Yet my code does not work with <code>stri_replace_all</code>, the corresponding <code>stringi</code> function.</p>
  
  <p>I am writing a quick regex to convert (a subset of) camel case to spaced words.</p>
  
  <p>I am quite puzzled as to why this works:</p>
  
  <pre><code>str &lt;- "thisIsCamelCase aintIt"
  stringr::str_replace_all(str, 
                           pattern="(?&lt;=[a-z])([A-Z])", 
                           replacement=" \\1")
  # "this Is Camel Case ain't It"
  </code></pre>
  
  <p>And this does not:</p>
  
  <pre><code>stri_replace_all(str, 
                   regex="(?&lt;=[a-z])([A-Z])", 
                   replacement=" \\1")
  # "this 1s 1amel 1ase ain't 1t"
  </code></pre>