title: Add 0 in a string of characters in R
body: <p>I have a dataset that has column wells A1-A24, B1-B24 all through P24. I would like to add a 0 to the middle of any wells with only 2 characters. For example, A1 would become A01 and B24 would stay B24.</p>
  <p>This is an example of what I have:</p>
  <p>data.frame</p>
  <pre><code>1   A1
  2   A2
  3   A3
  4   A4
  5   A5
  6   A6
  7   A7
  8   A8
  9   A9
  10  A10
  11  A11
  12  A12
  </code></pre>
  <p>This is what I want:</p>
  <p>data.frame</p>
  <pre><code>1   A01
  2   A02
  3   A03
  4   A04
  5   A05
  6   A06
  7   A07
  8   A08
  9   A09
  10  A10
  11  A11
  12  A12
  </code></pre>