in order to use some of the regular expressions in these examples remember to use the switch/option -E or -r to use extended regular expressions
Regular Expressions groups, the whole match will be in group 0 that can be referred by $0 or \0 depends on the programming/scripting language used for the regular expression, the first ( ) in the regular expression will be $1, the second ( ) $2 and so on.
here is an example of a text file with a list of names, and we want to change it from lastname, firstname to firstname, lastname
the regex match information picture if from https://regex101.com/ , where you can test your regex
Caulkins, Thomas Frazier, Crissy Jourdan, Lavera Birnbaum, Diedre Briscoe, Yessenia
cat example.txt | sed -E 's/(.*),[[:space:]](.*)/\2, \1/g' Thomas, Caulkins Crissy, Frazier Lavera, Jourdan Diedre, Birnbaum Yessenia, Briscoe
another example on a regex with groups String: 00298 311234 regex: (\d+)[[:space:]](\d+)