title: regexp to include underscore
body: <p>I'm new to using to regexp. I have the following: </p>
  
  <pre><code>  files = c("apple_2014_10_17.csv",
            "apple_2014_10_18.csv",
            "applepie_2014_10_17.csv",
            "applepie_2014_10_18.csv")
  </code></pre>
  
  <p>I am looking to return <strong>only</strong>:</p>
  
  <pre><code>apple_2014_10_17.csv
  apple_2014_10_18.csv
  </code></pre>
  
  <p>and <strong>NOT</strong> return:</p>
  
  <pre><code>applepie_2014_10_17.csv
  applepie_2014_10_18.csv
  </code></pre>
  
  <p>I'm using the following regexp </p>
  
  <pre><code>grepl("apple_*", files)
  </code></pre>
  
  <p>But it returns all the files. Any assistance would be greatly appreciated.</p>