Replace-regex Add string to string (Storing matches in variables)

This is the string: my_string2

M-x replace-regexp RET \(my_string[0-9]\) RET # What’s included inside () is stored in variables. The first match, stored in \1, second in \2 and so on. \1SoCool

This is the result: my_string2SoCool

https://www.emacswiki.org/emacs/RegularExpression https://stackoverflow.com/questions/2003774/emacs-how-to-replace-a-string-using-a-regular-expression