Until now, we've seen grep used with various options that alter its behavior. There is one final important option we'd like to share with you: --extended-regexp (-E). As the man grep page states, this means interpret PATTERN as an extended regular expression.
In contrast to the default regular expressions found in Linux, extended regular expressions have search patterns that are a lot closer to regular expressions in other scripting/programming languages (should you already have experience with those).
Specifically, the following constructs are available when using extended regular expressions over default regular expressions:
|
? |
Matches a repeat of the previous character zero or more times |
|
+ |
Matches a repeat ... |
