title: Why won't this regexp work in google spreadsheets?
body: <p>I'm trying to extract from a url using a regexp in google spreadsheets.  However the spreadsheet returns <strong><code>#VALUE!</code></strong> with the following error:  <strong>Invalid regular expression: invalid perl operator: (?&lt;</strong></p>
  
  <p>Here is the regexp I'm using: <code>(?&lt;=raid_boss=)[a-zA-Z0-9_]+</code></p>
  
  <p>A sample url will contain a variable in it that says <code>raid_boss=name</code>.  This regexp should extract name.  It works in my testing program, but not in google spreadsheet.  </p>
  
  <p>Here is the exact contents of the cell in google spreadsheets: <code>=REGEXEXTRACT( B1 ; "/(?&lt;=raid_boss=)[-a-zA-{}-9_]+" )</code></p>
  
  <p>Any insight or help would be much appreciated, thank you!</p>