Update fields that contains certain prefix by lookup table
Typically the pattern would be something like this:

UPDATE t
  SET t.ColumnWithName1 = 'DOMAIN\\' + new_old.NAMENEW
  FROM dbo.Table1 AS t
  INNER JOIN #NAMEOLD_NAMENEW AS new_old
  ON t.ColumnWithName1 = new_old.NAMEOLD
  WHERE &lt;some filter on a pattern&gt;;


But your question doesn't really explain how to identify the specific columns that match. Is [/1] a real string value, a pattern, or something else? Is \(.*) supposed to be a regular expression (which doesn't exist in native T-SQL)? Is the User column a typo or do you have multiple columns that have this domain prefix?