If you have this data : "01/01/2020"
With this pattern : "([0-3][0-9])/([0-1][0-9])/([0-2][0-9][0-9][0-9])"
You can only extract data by using the number of order of the different groups you made : "3-2-1" to extract this : "2020-01-01"
The official PERL documentation (which is the one recommanded by Safe to look at for REGEX) allows you to use names for groups for an easier calling.
So, with the previous data : "01/01/2020"
You could use this pattern : "(?<day>[0-3][0-9])/(?<month>[0-1][0-9])/(?<year>[0-2][0-9][0-9][0-9])"
And extract data by using this call : "g{year}-g{month}-g{day}" to extract this : "2020-01-01".
This exemple is really simple but when you have tenth of group for a very complexe REGEX search, it becomes very difficult to use.
REGEX doc on Safe website : http://docs.safe.com/fme/html/FME_Desktop_Documentation/FME_Transformers/!Transformer_Parameters/Regular-Expression-Editor.htm
PERL REGEX doc recommended by Safe : https://perldoc.perl.org/perlre.html