# - Print count register.

# displays the contents of the count register. The count register is affected by a number of commands.

After an S command, the count register is set to the number of substitutions made by the command.
After a G command, the count register contains the number of lines marked for processing by the given command.
After an R, W, or L command, the count register contains the number of blocks read or written or listed. If an access error occurred in the I/O operation, the count register will contain the file system error status.
Whenever a number register is changed by an N command, the count register is set to the new value of that number register.

To give an example of how # can be used, we can print the number of characters on a line (excluding the new-line character) with the instruction

s/./&/#

The command

*s/./&/#

prints the number of characters in the entire buffer (excluding the new-lines).