Brackets in bash

Are used to match a character from range:
[[:upper:]] or [A-Z]
[[:lower:]] or [a-z]
[[:digit:]] or [0-9]
[[:alpha:]] or [a-zA-Z]
[[:alphanum:]] or [a-zA-Z0-9]

Example:
$ ls file_[[:digit:]]
file_1  file_2  file_3  file_4