title: Getting substring by index position
body: <p>i need to extract a substring from a line using the initial and final position. I think it should be easily done with grep, but still haven't figured out how.</p>
  
  <p>An example, a line of n chars, i want to extract the substring starting at k position and ending in the l position of that line.</p>
  
  <p>With obviously l, k &lt; n and l > k</p>