title: Retain elements in array if associative key starts with specific substring
body: <p>If I have array like:</p>
  <pre><code>array [
         y =&gt; 35
         x =&gt; 51
         z =&gt; 35
         c_3 =&gt; 4
         c_1 =&gt; 54
         c_6 =&gt; 53
         c_9 =&gt; 52
  ] 
  </code></pre>
  <p>I want to get array of:</p>
  <pre><code>array [c_3=&gt;4, c_1=&gt;54, c_6=&gt;53, c_9=&gt;52]
  </code></pre>
  <p>How can I filter out the elements which do not have keys starting with <code>c_</code>?</p>