elisp: String Basics
4.1 String and Character Basics
===============================
A character is a Lisp object which represents a single character of
text. In Emacs Lisp, characters are simply integers; whether an integer
is a character or not is determined only by how it is used. 
Character Codes, for details about character representation in Emacs.
A string is a fixed sequence of characters. It is a type of sequence
called a “array”, meaning that its length is fixed and cannot be altered
once it is created (Sequences Arrays Vectors). Unlike in C,
Emacs Lisp strings are _not_ terminated by a distinguished character
code.
Since strings are arrays, and therefore sequences as well, you can
operate on them with the general array and sequence functions documented
in Sequences Arrays Vectors. For example, you can access or
change individual characters in a string using the functions ‘aref’ and
‘aset’ (Array Functions). However, note that ‘length’ should
_not_ be used for computing the width of a string on display; use
‘string-width’ (Size of Displayed Text) instead.
There are two text representations for non-ASCII characters in Emacs
strings (and in buffers): unibyte and multibyte. For most Lisp
programming, you don’t need to be concerned with these two
representations. Text Representations, for details.
Sometimes key sequences are represented as unibyte strings. When a
unibyte string is a key sequence, string elements in the range 128 to
255 represent meta characters (which are large integers) rather than
character codes in the range 128 to 255. Strings cannot hold characters
that have the hyper, super or alt modifiers; they can hold ASCII control
characters, but no other control characters. They do not distinguish
case in ASCII control characters. If you want to store such characters
in a sequence, such as a key sequence, you must use a vector instead of
a string. Character Type, for more information about keyboard
input characters.
Strings are useful for holding regular expressions. You can also
DONTPRINTYET match regular expressions against strings with ‘string-match’ (
Regexp Search). The functions ‘match-string’ (*noteSimple Match
Data::) and ‘replace-match’ (Replacing Match) are useful for
decomposing and modifying strings after matching regular expressions
against them.
Like a buffer, a string can contain text properties for the
characters in it, as well as the characters themselves. Text
Properties. All the Lisp primitives that copy text from strings to
buffers or other strings also copy the properties of the characters
being copied.
Text, for information about functions that display strings or
DONTPRINTYET copy them into buffers. Character Type, and *noteString
DONTPRINTYET copy them into buffers. Character Type, and String
Type, for information about the syntax of characters and strings.
Non-ASCII Characters, for functions to convert between text
representations and to encode and decode character codes.
Info Catalog
elisp: Strings and Characters
elisp: Predicates for Strings