title: Using str_match with lookbehind (U_REGEX_LOOK_BEHIND_LIMIT)
body: <p>I'm trying to use regex to extract a string, it works here: <a href="https://regexr.com/3vsd4" rel="nofollow noreferrer">https://regexr.com/3vsd4</a></p>
  
  <p>But when running something similar in R it fails: </p>
  
  <pre><code>m =  "(?&lt;=~* )([ AP_])\\w+"
  x = "XY_O ~ R_Z + YPP_L_WINTER + AP_C"
  str_match(x, m)[1, 1]
  </code></pre>
  
  <p>Gives the error:</p>
  
  <blockquote>
    <p>Error in stri_match_first_regex(string, pattern, opts_regex = opts(pattern)) : 
    Look-Behind pattern matches must have a bounded maximum length. (U_REGEX_LOOK_BEHIND_LIMIT)</p>
  </blockquote>
  
  <p>How can I edit the <code>R</code> code to return <code>"AP_C"</code>?</p>