title: Editing a file by adding a prefix at specific places
body: <p>I have a file which looks like this:</p>
  
  <pre><code>fixedStep chrom=20 start=1 step=1000   
  0   
  10   
  20   
  100   
  0  
  fixedStep chrom=19 start=1 step=1000   
  0   
  0   
  50   
  330   
  450   
  0
  </code></pre>
  
  <p>I want to add a prefix <code>chr</code> to the number which comes after <code>chrom=</code>. Therefor, instead of having <code>chrom=20</code> or <code>chrom=19</code>, I want to have <code>chrom=chr20</code>or <code>chrom=chr20</code>. So the file should look like this:</p>
  
  <pre><code>fixedStep chrom=chr20 start=1 step=1000   
  0   
  10   
  20   
  100   
  0  
  fixedStep chrom=chr19 start=1 step=1000   
  0   
  0   
  50   
  330   
  450   
  0
  </code></pre>
  
  <p>I know that with <code>sed</code>command I will be able to do this, but what I don't know is how to construct a <code>regex</code>which finds the <code>chrom=</code>part. I had already used <code>sed</code>to modify simple things like adding a prefix to the beginning of each line in a file; but I cannot wrap my mind around this question!</p>