(Character Type): Node split.
Add xref to Describing Characters. (Basic Char Syntax, General Escape Syntax) (Ctl-Char Syntax, Meta-Char Syntax): New subnodes.
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Richard M. Stallman
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1 changed files with 99 additions and 68 deletions
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@ -227,9 +227,9 @@ number whose value is 1500. They are all equivalent.
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other words, characters are represented by their character codes. For
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example, the character @kbd{A} is represented as the @w{integer 65}.
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Individual characters are not often used in programs. It is far more
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common to work with @emph{strings}, which are sequences composed of
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characters. @xref{String Type}.
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Individual characters are used occasionally in programs, but it is
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more common to work with @emph{strings}, which are sequences composed
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of characters. @xref{String Type}.
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Characters in strings, buffers, and files are currently limited to
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the range of 0 to 524287---nineteen bits. But not all values in that
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@ -239,17 +239,32 @@ range are valid character codes. Codes 0 through 127 are
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input have a much wider range, to encode modifier keys such as
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Control, Meta and Shift.
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There are special functions for producing a human-readable textual
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description of a character for the sake of messages. @xref{Describing
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Characters}.
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@menu
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* Basic Char Syntax::
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* General Escape Syntax::
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* Ctl-Char Syntax::
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* Meta-Char Syntax::
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* Other Char Bits::
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@end menu
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@node Basic Char Syntax
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@subsubsection Basic Char Syntax
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@cindex read syntax for characters
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@cindex printed representation for characters
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@cindex syntax for characters
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@cindex @samp{?} in character constant
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@cindex question mark in character constant
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Since characters are really integers, the printed representation of a
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character is a decimal number. This is also a possible read syntax for
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a character, but writing characters that way in Lisp programs is a very
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bad idea. You should @emph{always} use the special read syntax formats
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that Emacs Lisp provides for characters. These syntax formats start
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with a question mark.
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Since characters are really integers, the printed representation of
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a character is a decimal number. This is also a possible read syntax
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for a character, but writing characters that way in Lisp programs is
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not clear programming. You should @emph{always} use the special read
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syntax formats that Emacs Lisp provides for characters. These syntax
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formats start with a question mark.
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The usual read syntax for alphanumeric characters is a question mark
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followed by the character; thus, @samp{?A} for the character
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@ -315,8 +330,76 @@ the ``super'' modifier to the following character.) Thus,
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character @key{ESC}. @samp{\s} is meant for use in character
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constants; in string constants, just write the space.
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A backslash is allowed, and harmless, preceding any character without
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a special escape meaning; thus, @samp{?\+} is equivalent to @samp{?+}.
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There is no reason to add a backslash before most characters. However,
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you should add a backslash before any of the characters
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@samp{()\|;'`"#.,} to avoid confusing the Emacs commands for editing
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Lisp code. You can also add a backslash before whitespace characters such as
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space, tab, newline and formfeed. However, it is cleaner to use one of
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the easily readable escape sequences, such as @samp{\t} or @samp{\s},
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instead of an actual whitespace character such as a tab or a space.
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(If you do write backslash followed by a space, you should write
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an extra space after the character constant to separate it from the
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following text.)
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@node General Escape Syntax
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@subsubsection General Escape Syntax
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In addition to the specific excape sequences for special important
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control characters, Emacs provides general categories of escape syntax
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that you can use to specify non-ASCII text characters.
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@cindex unicode character escape
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For instance, you can specify characters by their Unicode values.
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@code{?\u@var{nnnn}} represents a character that maps to the Unicode
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code point @samp{U+@var{nnnn}}. There is a slightly different syntax
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for specifying characters with code points above @code{#xFFFF};
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@code{\U00@var{nnnnnn}} represents the character whose Unicode code
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point is @samp{U+@var{nnnnnn}}, if such a character is supported by
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Emacs. If the corresponding character is not supported, Emacs signals
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an error.
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This peculiar and inconvenient syntax was adopted for compatibility
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with other programming languages. Unlike some other languages, Emacs
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Lisp supports this syntax in only character literals and strings.
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@cindex @samp{\} in character constant
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@cindex backslash in character constant
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@cindex octal character code
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The most general read syntax for a character represents the
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character code in either octal or hex. To use octal, write a question
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mark followed by a backslash and the octal character code (up to three
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octal digits); thus, @samp{?\101} for the character @kbd{A},
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@samp{?\001} for the character @kbd{C-a}, and @code{?\002} for the
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character @kbd{C-b}. Although this syntax can represent any
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@acronym{ASCII} character, it is preferred only when the precise octal
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value is more important than the @acronym{ASCII} representation.
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@example
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@group
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?\012 @result{} 10 ?\n @result{} 10 ?\C-j @result{} 10
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?\101 @result{} 65 ?A @result{} 65
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@end group
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@end example
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To use hex, write a question mark followed by a backslash, @samp{x},
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and the hexadecimal character code. You can use any number of hex
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digits, so you can represent any character code in this way.
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Thus, @samp{?\x41} for the character @kbd{A}, @samp{?\x1} for the
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character @kbd{C-a}, and @code{?\x8e0} for the Latin-1 character
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@iftex
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@samp{@`a}.
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@end iftex
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@ifnottex
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@samp{a} with grave accent.
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@end ifnottex
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@node Ctl-Char Syntax
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@subsubsection Control-Character Syntax
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@cindex control characters
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Control characters may be represented using yet another read syntax.
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Control characters can be represented using yet another read syntax.
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This consists of a question mark followed by a backslash, caret, and the
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corresponding non-control character, in either upper or lower case. For
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example, both @samp{?\^I} and @samp{?\^i} are valid read syntax for the
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@ -363,6 +446,9 @@ input, we prefer the @samp{C-} syntax. Which one you use does not
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affect the meaning of the program, but may guide the understanding of
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people who read it.
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@node Meta-Char Syntax
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@subsubsection Meta-Character Syntax
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@cindex meta characters
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A @dfn{meta character} is a character typed with the @key{META}
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modifier key. The integer that represents such a character has the
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@ -395,6 +481,9 @@ syntax for a character. Thus, you can write @kbd{M-A} as @samp{?\M-A},
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or as @samp{?\M-\101}. Likewise, you can write @kbd{C-M-b} as
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@samp{?\M-\C-b}, @samp{?\C-\M-b}, or @samp{?\M-\002}.
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@node Other Char Bits
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@subsubsection Other Character Modifier Bits
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The case of a graphic character is indicated by its character code;
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for example, @acronym{ASCII} distinguishes between the characters @samp{a}
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and @samp{A}. But @acronym{ASCII} has no way to represent whether a control
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@ -431,64 +520,6 @@ Numerically, the
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bit values are 2**22 for alt, 2**23 for super and 2**24 for hyper.
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@end ifnottex
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@cindex unicode character escape
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Emacs provides a syntax for specifying characters by their Unicode
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code points. @code{?\u@var{nnnn}} represents a character that maps to
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the Unicode code point @samp{U+@var{nnnn}}. There is a slightly
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different syntax for specifying characters with code points above
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@code{#xFFFF}; @code{\U00@var{nnnnnn}} represents the character whose
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Unicode code point is @samp{U+@var{nnnnnn}}, if such a character
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is supported by Emacs. If the corresponding character is not
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supported, Emacs signals an error.
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This peculiar and inconvenient syntax was adopted for compatibility
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with other programming languages. Unlike some other languages, Emacs
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Lisp supports this syntax in only character literals and strings.
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@cindex @samp{\} in character constant
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@cindex backslash in character constant
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@cindex octal character code
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Finally, the most general read syntax for a character represents the
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character code in either octal or hex. To use octal, write a question
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mark followed by a backslash and the octal character code (up to three
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octal digits); thus, @samp{?\101} for the character @kbd{A},
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@samp{?\001} for the character @kbd{C-a}, and @code{?\002} for the
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character @kbd{C-b}. Although this syntax can represent any @acronym{ASCII}
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character, it is preferred only when the precise octal value is more
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important than the @acronym{ASCII} representation.
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@example
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@group
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?\012 @result{} 10 ?\n @result{} 10 ?\C-j @result{} 10
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?\101 @result{} 65 ?A @result{} 65
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@end group
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@end example
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To use hex, write a question mark followed by a backslash, @samp{x},
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and the hexadecimal character code. You can use any number of hex
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digits, so you can represent any character code in this way.
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Thus, @samp{?\x41} for the character @kbd{A}, @samp{?\x1} for the
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character @kbd{C-a}, and @code{?\x8e0} for the Latin-1 character
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@iftex
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@samp{@`a}.
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@end iftex
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@ifnottex
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@samp{a} with grave accent.
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@end ifnottex
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A backslash is allowed, and harmless, preceding any character without
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a special escape meaning; thus, @samp{?\+} is equivalent to @samp{?+}.
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There is no reason to add a backslash before most characters. However,
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you should add a backslash before any of the characters
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@samp{()\|;'`"#.,} to avoid confusing the Emacs commands for editing
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Lisp code. You can also add a backslash before whitespace characters such as
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space, tab, newline and formfeed. However, it is cleaner to use one of
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the easily readable escape sequences, such as @samp{\t} or @samp{\s},
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instead of an actual whitespace character such as a tab or a space.
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(If you do write backslash followed by a space, you should write
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an extra space after the character constant to separate it from the
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following text.)
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@node Symbol Type
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@subsection Symbol Type
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