title: regex negative lookahead - exclude the exact word
body: <p>I need to modify an identifier, and it has a certain first letter, and zero or more continue letters, and I want to exclude some reserved words.
  For the example I'm using the same first and continue letters:</p>
  
  <pre><code>(?!(abstract|alignof|as|impl|in|let|mut))[a-z][a-z]*
  </code></pre>
  
  <p>so abstract|alignof|as|impl|in|let|mut are the words I don't want to match, the thing is, with this regex I cant match the word "letter" because of "let". it only matches "etter"</p>
  
  <p>I also tried this:</p>
  
  <pre><code>(?!(abstract|alignof|as|impl|in|let|mut))\b[a-z][a-z]*\b
  </code></pre>
  
  <p>but it doesn't seem to work.
  How can I exclude the exact word?</p>
  
  <p>Thanks</p>