What are regular expressions, and how do you work with them in R?

A regular expression, or regex, in R or other programming languages, is a character or a sequence of characters that describes a certain text pattern and is used for mining text data. In R, there are two main ways of working with regular expressions:

Using the base R and its functions (such as grep(), regexpr(), gsub(), regmatches(), etc.) to locate, match, extract, and replace regex.
Using a specialized stringr package of the tidyverse collection. This is a more convenient way to work with R regex since the functions of stringr have much more intuitive names and syntax and offer more extensive functionality.