title: Pandas dataframe search and modify leading zero on postal codes
body: <p>I have a pandas DataFrame database of location and postal code addresses. But postal codes have been interpreted from Excel reading. For example, in France postal addresses are a department code followed by city code, e.g. 75600 (75 = 'Paris region', 600 = 'inner city').</p>
  <p>But in some first postal codes, e.g. 01200 it have interpreted like 1200. How can I search integer values below 10000 and modify them? Or how to preserve the first zero. How to search and replace in a dataframe and use the content (to modify it)?</p>