Learn Linux Shell Scripting - Fundamentals of Bash 4.4

Character classes

We've now seen many examples of how to use regular expressions. While most things are pretty intuitive, we have also seen that if we want to filter for both uppercase and lowercase strings, we'd either have to specify the -i option for grep, or change the search pattern from [a-z] to [a-zA-z]. For numbers, we would need to use [0-9].

Some might find this fine to work with, but others might disagree. In this case, there is an alternative notation that can be used: [[:pattern:]].

The next example uses both this new double bracket notation, and the old single bracket one:

reader@ubuntu:~/scripts/chapter_10$ grep [[:digit:]] character-class.txt e2ea2areader@ubuntu:~/scripts/chapter_10$ grep [0-9] character-class.txt e2ea2a