How to enter non-ascii characters using hex or octal codes in vi

:help i_CTRL-V_digit

In insert mode, type Ctrl+V followed by

• a decimal number (0-255)
• o then an octal number (o0-o377, i.e., 255 is the maximum value)
• x then a hex number (x00-xFF, i.e., 255 is the maximum value)
• u then a 4-hexchar Unicode sequence
• U then an 8-hexchar Unicode sequence

Decimal and octal numbers are limited to three digits.  Decimal numbers less than 100 may include leading zeroes, which are ignored.  Octal numbers less than 100_oct (i.e., 64) may include leading zeroes, but they are not required.  Octal numbers greater than or equal to 100_oct may not include leading zeroes (but you may type a leading o if you want to).

You can terminate a number by typing a character that is not a valid digit for that radix.  For example,

• Ctrl+V    0 6 5 → A.
• Ctrl+V    6 5 B → Ab.
• Ctrl+V o  0 4 1 → !.
• Ctrl+V o  4 1 9 → !9.

Regular (one-octet) hex numbers are limited to two digits.  As with the above, you can repeat the radix character (e.g., Ctrl+V u u  0 0 4 1 → A)  for characters specified by hex codes.  o and x are case-insensitive.

I assume that you use vim, because :helpoctal is a vim's command. On some systems vi is just a symlink to vim which runs it in vi-compatible mode.

In vim:

• You can enter unicode characters from basic multilingual plane you can use:
  Press ctrl+v and then enter four digit hex unicode code.
• Another option is digraphs. You can read more about them in vim's help (help: dig).
  Press ctrl+k and then two-character sequence.
  You can list sequences supported in you vim usig command :digraph .

In nvi, vi and elsewhere:

• Ctrl+Shift and hit U and then enter unicode hex code.