title: What are the rules for a valid variable name in mirth?
body: <p>I am trying to set up a transformer on a Database Reader to file writer channel. I am reading in a sql field called MRN which I would like to send to a variable called mrn. I added a step to a channel with a variable called tmp['MSH'] mapping to a variable called msg['MSH'] But mirth is giving me the error message: </p>
  
  <p><em>The variable name contains invalid characters. Please enter a new variable name</em></p>
  
  <p>What are the rules for a valid variable name in mirth?</p>