elisp: Non-ASCII in Strings
2.3.8.2 Non-ASCII Characters in Strings
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There are two text representations for non-ASCII characters in Emacs
strings: multibyte and unibyte (
Text Representations). Roughly
speaking, unibyte strings store raw bytes, while multibyte strings store
human-readable text. Each character in a unibyte string is a byte,
i.e., its value is between 0 and 255. By contrast, each character in a
multibyte string may have a value between 0 to 4194303 (Character
Type). In both cases, characters above 127 are non-ASCII.
You can include a non-ASCII character in a string constant by writing
it literally. If the string constant is read from a multibyte source,
such as a multibyte buffer or string, or a file that would be visited as
multibyte, then Emacs reads each non-ASCII character as a multibyte
character and automatically makes the string a multibyte string. If the
string constant is read from a unibyte source, then Emacs reads the
non-ASCII character as unibyte, and makes the string unibyte.
Instead of writing a character literally into a multibyte string, you
can write it as its character code using an escape sequence.
General Escape Syntax, for details about escape sequences.
If you use any Unicode-style escape sequence ‘\uNNNN’ or ‘\U00NNNNNN’
in a string constant (even for an ASCII character), Emacs automatically
assumes that it is multibyte.
You can also use hexadecimal escape sequences (‘\xN’) and octal
escape sequences (‘\N’) in string constants. *But beware:* If a string
constant contains hexadecimal or octal escape sequences, and these
escape sequences all specify unibyte characters (i.e., less than 256),
and there are no other literal non-ASCII characters or Unicode-style
escape sequences in the string, then Emacs automatically assumes that it
is a unibyte string. That is to say, it assumes that all non-ASCII
characters occurring in the string are 8-bit raw bytes.
In hexadecimal and octal escape sequences, the escaped character code
may contain a variable number of digits, so the first subsequent
character which is not a valid hexadecimal or octal digit terminates the
escape sequence. If the next character in a string could be interpreted
as a hexadecimal or octal digit, write ‘\ ’ (backslash and space) to
terminate the escape sequence. For example, ‘\xe0\ ’ represents one
character, ‘a’ with grave accent. ‘\ ’ in a string constant is just
like backslash-newline; it does not contribute any character to the
string, but it does terminate any preceding hex escape.
Info Catalog
elisp: Syntax for Strings
elisp: String Type
elisp: Nonprinting Characters