title: Extracting range of unpadded string
body: <p>I'd like to extract the <code>Range&lt;String.Index&gt;</code> of a sentence within its whitespace padding. For example,</p>
  
  <pre><code>let padded = "    El águila ().  " 
  let sentenceRangeInPadded = ???
  assert(padded[sentenceRangeInPadded] == "El águila ().") // The test!
  </code></pre>
  
  <p>Here's some regex that I started with, but looks like variable length lookbehinds aren't supported.</p>
  
  <pre><code>let sentenceRangeInPadded = padded.range(of: #"(?&lt;=^\s*).*?(?=\s*$)"#, options: .regularExpression)!
  </code></pre>
  
  <p>I'm not looking to extract the sentence (could just use <code>trimmingCharacters(in:)</code> for that), just the <code>Range</code>.</p>
  
  <p>Thanks for reading!</p>