32.1.1 Event Designators

An event designator is a reference to a command line entry in the history list.

Start a history substitution, except when followed by a space, tab, the end of the line, or =.
Refer to command line n.
Refer to the command n lines back.
Refer to the previous command. This is a synonym for !-1.
Refer to the most recent command starting with string.
Refer to the most recent command containing string. The trailing ? may be omitted if the string is followed immediately by a newline.
Quick Substitution. Repeat the last command, replacing string1 with string2. Equivalent to !!:s/string1/string2/.
The entire command line typed so far.