17.4 Predefined variables to use in regular expressions:

[:lower:]	Lower-case letters
[:upper:]	Upper-case letters
[:alpha:]	Alphabetic characters: [:lower:] and [:upper:]
[:digit:]	Digits: 0 1 2 3 4 5 6 7 8 9
[:alnum:]	Alphanumeric characters: [:alpha:] and [:digit:]
[:print:]	Printable characters: [:alnum:], [:punct:] and space.
[:punct:]	Punctuation characters: ! " # $ % & ’ ( ) * + , - . / : ; < = > ? @ [  ] ^ _ ` { | } ~
[:blank:]	Blank characters: space and tab

• Take the previous character vector containing email addresses:
  • Remove the @ and the email provider from each address

gsub(pattern="@[[:lower:][:punct:]]+", 
    replacement="", 
    x=vec_ad)

• Same thing but remove additionally any number(s) BEFORE the @ (if any):

gsub(pattern="[[:digit:]]*@[[:lower:][:punct:]]+",
        replacement="",
        x=vec_ad)

• Same but simplified:

gsub(pattern="[[:digit:]]*@[[:print:]]+",
        replacement="",
        x=vec_ad)