Unless you are already familiar with regular expressions, the discussion of special characters above probably looks forbiddingly complex. A few more examples should make things clearer. In the examples that follow, a square () is used to mark a space; it is not a special character.
Let's work through how you might use some special characters in a replacement. Suppose that you have a long file and that you want to substitute the word child with the word children throughout that file. You first save the edited buffer with :w, then try the global replacement:
When you continue editing, you notice occurrences of words such as childrenish. You have unintentionally matched the word childish. Returning to the last saved buffer with :e!, you now try:
(Note that there is a space after child.) But this command misses the occurrences child., child,, child: and so on. After some thought, you remember that brackets allow you to specify one character from among a list, so you realize a solution: