Replace the first character of each line

Question

In vi, how to replace the first character in each line if it is the specified character a with another character !? If the first character is space，do not do anything.

Answer 1

Global substitute the first character of a line if it is 'a':

:%s/^[a]/!/

Global substitute the first character of a line if it is not space:

:%s/^[^ ]/!/

Answer 2

Or using global command :g

:g/^a/s//!

this will automatically skip all lines that begin with a space or a character that does not match ^a and replace all matching with !