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(elisp)Character Codes

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Character Codes
===============

   The unibyte and multibyte text representations use different
character codes.  The valid character codes for unibyte representation
range from 0 to 255--the values that can fit in one byte.  The valid
character codes for multibyte representation range from 0 to 524287,
but not all values in that range are valid.  The values 128 through 255
are not entirely proper in multibyte text, but they can occur if you do
explicit encoding and decoding (Note: Explicit Encoding).  Some other
character codes cannot occur at all in multibyte text.  Only the ASCII
codes 0 through 127 are completely legitimate in both representations.

 - Function: char-valid-p charcode &optional genericp
     This returns `t' if CHARCODE is valid for either one of the two
     text representations.

          (char-valid-p 65)
               => t
          (char-valid-p 256)
               => nil
          (char-valid-p 2248)
               => t

     If the optional argument GENERICP is non-nil, this function
     returns `t' if CHARCODE is a generic character (Note: Splitting
     Characters).