^Only$ matches a line that consists solely of the word Only—no spaces, no other characters, nothing. Only Only is allowed.
b[aou]rn matches barn, born, burn.
Ver[D-F] matches VerD, VerE, VerF.
Ver[^0-9] matches Ver followed by any non-digit.
the[ir][re] matches their, therr, there, theie.
[A-Za-z][A-Za-z]* matches any word which consists of only letters, and at least one letter. It will not match numbers or spaces.
13.2 Advanced Files
Now that you have a basic understanding of files, it is time to learn more advanced things about them.
13.2.1 The Real Nature of Files: Hard Links and Inodes
Each file on your system is represented by an inode (for Information Node; pronounced “eye-node”). An inode contains all the information about the file. However, the inode is not directly visible. Instead, each inode is linked into the filesystem by one or more hard links. Hard links contain the name of the file and the inode number. The inode contains the file itself, i.e., the location of the information being stored on disk, its access permissions, the file type, and so on. The system can find any inode if it has the inode number.