title: LUA: how to find and match a specific string in a file name
body: <p>I have a file named medicine_NU12345.pdf. I want to extract the NU12345 from the pdf filename. How can I do this using Lua.</p>
  
  <p>I am able to do a string.find('NU') but how do I include it to find the numbers. The numbers change for each file</p>