title: MARC21 - bibliographic record file: script to create new fields and put there modified values
body: <p>I have a MARC-file with bibliographic records which consist of different fields that have values. e.g. Author, Title, Publication year etc. The data structure looks like this: </p>
  
  <pre><code>700  1   |a King, Robert Harlen 
  852  L   |3 81TYD2565(2) 
  852  L   |3 Ugk25 1:2B 
  </code></pre>
  
  <p>To implement a certain feature I need e.g. the value of field 852|3 copied, modified and written in the record-file like this:</p>
  
  <pre><code>852  L   |4 TYD
  852  L   |4 Ugk25
  </code></pre>
  
  <p>As I am new to this I would like to know if someone has an idea what would be the best approach to do that?</p>
  
  <p>ps: the MARC-file will be indexed into a SolR-index. Is there a possibility to make the changes during the indexing?</p>