(Builds on: String basics)
Regular expressions are a extremely concise language for describing patterns in strings. They are important to master because they unlock a number of new tools that allow you to bend strings to your will.
Matching patterns with regular expressions [r4ds-14.3]
Tools [r4ds-14.4]
String manipulation with stringr cheatsheet [cheatsheets-strings]. The Regular Expressions section is a very helpful reference.