title: awk to extract lines in file that contain matching pattern and variable digit
body: <p>I am trying to use <code>awk</code> to extract those lines that in <code>$2</code> contain <code>exon (some digit that is 1-99) sequence</code>. The text will always be the same but the digit will be variable.</p>
  
  <p><strong>file</strong> <code>tab-delimeted</code></p>
  
  <pre><code>Tier 2  exon 10 sequence    xxxxx
  Tier 2  full sequence   yyyyy
  Tier 1  exon 5 sequence aaaaa
  </code></pre>
  
  <p><strong>desired output</strong> <code>tab-delimeted</code></p>
  
  <pre><code>Tier 2  exon 10 sequence    xxxxx
  Tier 1  exon 5 sequence aaaaa
  </code></pre>
  
  <p><strong>awk</strong></p>
  
  <pre><code> awk '$2 ~ /^exon [0-9][0-9] sequence$/' file
  </code></pre>