ent is provided to <code>by</code>, it needs to be either 1) a list of column names, or 2) a vector of regular expressions. The format requirements are the following:</p><ol><li><p>In case a <strong>list</strong> is passed, each element of the list must be a vector of length equal to the number of tables being merged (i.e., if 3 tables are being merged, the list needs to contain all vectors of length 3). The vectors should contain the names of columns to be merged in each table, <code>NA</code> can be inser