> print(db_ref$get())
<taxon_db_def[8]>
   _____________________ ncbi ______________________
   url:         http://www.ncbi.nlm.nih.gov/taxonomy
   desc:        NCBI Taxonomy Database              
   id_regex:    [0-9]+                              
   rank_levels:                                     

   _____________________________ gbif _____________________________
   url:         http://www.gbif.org/developer/species              
   desc:        GBIF Taxonomic Backbone                            
   id_regex:    [0-9]+                                             
   rank_levels: kingdom < phylum < order < family < genus < species

   _________________________________ bold _________________________________
   url:         http://www.boldsystems.org                                 
   desc:        Barcode of Life                                            
   id_regex:    [0-9]+                                                     
   rank_levels: phylum < class < order < family < subfamily < genus < spec…

   ___________________ col ___________________
   url:         http://www.catalogueoflife.org
   desc:        Catalogue of Life             
   id_regex:    [a-z0-9]{32}                  
   rank_levels:                               

   ______________ eol ______________
   url:         http://eol.org      
   desc:        Encyclopedia of Life
   id_regex:    [0-9]+              
   rank_levels:                     

   ____________________ nbn ____________________
   url:         https://nbn.org.uk              
   desc:        UK National Biodiversity Network
   id_regex:    [A-Z]{5}[0-9]{10}               
   rank_levels:                                 

   ________________ tps ________________
   url:         http://www.tropicos.org/
   desc:        Tropicos                
   id_regex:    [0-9]+                  
   rank_levels:                         

   _______________________ itis _______________________
   url:         http://www.itis.gov                    
   desc:        Integrated Taxonomic Information System
   id_regex:    [0-9]+                                 
   rank_levels:                                        

