A string in Emacs Lisp is an array that contains an ordered sequence of characters. Strings are used as names of symbols, buffers, and files, to send messages to users, to hold text being copied between buffers, and for many other purposes. Because strings are so important, Emacs Lisp has many functions expressly for manipulating them. Emacs Lisp programs use strings more often than individual characters.
See section Putting Keyboard Events in Strings, for special considerations for strings of keyboard character events.
Strings in Emacs Lisp are arrays that contain an ordered sequence of characters. Characters are represented in Emacs Lisp as integers; whether an integer is a character or not is determined only by how it is used. Thus, strings really contain integers.
The length of a string (like any array) is fixed, and cannot be altered once the string exists. Strings in Lisp are not terminated by a distinguished character code. (By contrast, strings in C are terminated by a character with ASCII code 0.)
Since strings are arrays, and therefore sequences as well, you
can operate on them with the general array and sequence functions.
(See section Sequences, Arrays, and
Vectors.) For example, you can access or change individual
characters in a string using the functions aref and
aset (see section Functions
that Operate on Arrays).
There are two text representations for non-ASCII characters in Emacs strings (and in buffers): unibyte and multibyte (see section Text Representations). ASCII characters always occupy one byte in a string; in fact, there is no real difference between the two representation for a string which is all ASCII. For most Lisp programming, you don't need to be concerned with these two representations.
Sometimes key sequences are represented as strings. When a string is a key sequence, string elements in the range 128 to 255 represent meta characters (which are extremely 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. See section Character Type, for more information about representation of meta and other modifiers for keyboard input characters.
Strings are useful for holding regular expressions. You can also
match regular expressions against strings (see section Regular Expression Searching). The
functions match-string (see section Simple Match Data Access) and
replace-match (see section Replacing the Text That Matched) are
useful for decomposing and modifying strings based on regular
expression matching.
Like a buffer, a string can contain text properties for the characters in it, as well as the characters themselves. See section 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.
See section Text, for information about functions that display strings or copy them into buffers. See section Character Type, and section String Type, for information about the syntax of characters and strings. See section Non-ASCII Characters, for functions to convert between text representations and encode and decode character codes.
For more information about general sequence and array predicates, see section Sequences, Arrays, and Vectors, and section Arrays.
t if object is a string, nil
otherwise.
t if object is a string or a character
(i.e., an integer), nil otherwise.
The following functions create strings, either from scratch, or by putting strings together, or by taking them apart.
(make-string 5 ?x)
=> "xxxxx"
(make-string 0 ?x)
=> ""
Other functions to compare with this one include
char-to-string (see section Conversion of Characters and Strings),
make-vector (see section Vectors), and make-list
(see section Building Cons Cells and
Lists).
(string ?a ?b ?c)
=> "abc"
(substring "abcdefg" 0 3)
=> "abc"
Here the index for `a' is 0, the index for
`b' is 1, and the index for `c' is 2.
Thus, three letters, `abc', are copied from the string
"abcdefg". The index 3 marks the character position up
to which the substring is copied. The character whose index is 3 is
actually the fourth character in the string.
A negative number counts from the end of the string, so that -1 signifies the index of the last character of the string. For example:
(substring "abcdefg" -3 -1)
=> "ef"
In this example, the index for `e' is -3, the index for `f' is -2, and the index for `g' is -1. Therefore, `e' and `f' are included, and `g' is excluded.
When nil is used as an index, it stands for the
length of the string. Thus,
(substring "abcdefg" -3 nil)
=> "efg"
Omitting the argument end is equivalent to specifying
nil. It follows that (substring
string 0) returns a copy of all of
string.
(substring "abcdefg" 0)
=> "abcdefg"
But we recommend copy-sequence for this purpose
(see section Sequences).
If the characters copied from string have text properties, the properties are copied into the new string also. See section Text Properties.
substring also allows vectors for the first
argument. For example:
(substring [a b (c) "d"] 1 3)
=> [b (c)]
A wrong-type-argument error is signaled if either
start or end is not an integer or
nil. An args-out-of-range error is
signaled if start indicates a character following
end, or if either integer is out of range for
string.
Contrast this function with buffer-substring (see
section Examining Buffer Contents),
which returns a string containing a portion of the text in the
current buffer. The beginning of a string is at index 0, but the
beginning of a buffer is at index 1.
concat receives no
arguments, it returns an empty string.
(concat "abc" "-def")
=> "abc-def"
(concat "abc" (list 120 121) [122])
=> "abcxyz"
;; nil is an empty sequence.
(concat "abc" nil "-def")
=> "abc-def"
(concat "The " "quick brown " "fox.")
=> "The quick brown fox."
(concat)
=> ""
The concat function always constructs a new string
that is not eq to any existing string.
When an argument is an integer (not a sequence of integers), it
is converted to a string of digits making up the decimal printed
representation of the integer. Don't use this feature; we
plan to eliminate it. If you already use this feature, change your
programs now! The proper way to convert an integer to a
decimal number in this way is with format (see section
Formatting Strings) or
number-to-string (see section Conversion of Characters and
Strings).
(concat 137)
=> "137"
(concat 54 321)
=> "54321"
For information about other concatenation functions, see the
description of mapconcat in section Mapping Functions,
vconcat in section Vectors, and append in
section Building Cons Cells and
Lists.
nil (or
omitted), the default is "[ \f\t\n\r\v]+". For example,
(split-string "Soup is good food" "o")
=> ("S" "up is g" "" "d f" "" "d")
(split-string "Soup is good food" "o+")
=> ("S" "up is g" "d f" "d")
When there is a match adjacent to the beginning or end of the string, this does not cause a null string to appear at the beginning or end of the list:
(split-string "out to moo" "o+")
=> ("ut t" " m")
Empty matches do count, when not adjacent to another match:
(split-string "Soup is good food" "o*")
=>("S" "u" "p" " " "i" "s" " " "g" "d" " " "f" "d")
(split-string "Nice doggy!" "")
=>("N" "i" "c" "e" " " "d" "o" "g" "g" "y" "!")
The most basic way to alter the contents of an existing string
is with aset (see section Functions that Operate on Arrays).
(aset string idx
char) stores char into
string at index idx. Each character occupies
one or more bytes, and if char needs a different number
of bytes from the character already present at that index,
aset signals an error.
A more powerful function is store-substring:
Since it is impossible to change the length of an existing string, it is an error if obj doesn't fit within string's actual length, of if any new character requires a different number of bytes from the character currently present at that point in string.
t if the arguments represent the same character,
nil otherwise. This function ignores differences in
case if case-fold-search is non-nil.
(char-equal ?x ?x)
=> t
(let ((case-fold-search nil))
(char-equal ?x ?X))
=> nil
t if the characters of the two strings match exactly;
case is significant.
(string= "abc" "abc")
=> t
(string= "abc" "ABC")
=> nil
(string= "ab" "ABC")
=> nil
The function string= ignores the text properties of
the two strings. When equal (see section Equality Predicates) compares two
strings, it uses string=.
If the strings contain non-ASCII characters, and one is unibyte while the other is multibyte, then they cannot be equal. See section Text Representations.
string-equal is
another name for string=.
t. If the lesser character is the one
from string2, then string1 is greater, and
this function returns nil. If the two strings match
entirely, the value is nil. Pairs of characters are compared according to their character codes. Keep in mind that lower case letters have higher numeric values in the ASCII character set than their upper case counterparts; digits and many punctuation characters have a lower numeric value than upper case letters. An ASCII character is less than any non-ASCII character; a unibyte non-ASCII character is always less than any multibyte non-ASCII character (see section Text Representations).
(string< "abc" "abd")
=> t
(string< "abd" "abc")
=> nil
(string< "123" "abc")
=> t
When the strings have different lengths, and they match up to
the length of string1, then the result is
t. If they match up to the length of
string2, the result is nil. A string of no
characters is less than any other string.
(string< "" "abc")
=> t
(string< "ab" "abc")
=> t
(string< "abc" "")
=> nil
(string< "abc" "ab")
=> nil
(string< "" "")
=> nil
string-lessp is
another name for string<.
The strings are both converted to multibyte for the comparison
(see section Text Representations)
so that a unibyte string can be equal to a multibyte string. If
ignore-case is non-nil, then case is
ignored, so that upper case letters can be equal to lower case
letters.
If the specified portions of the two strings match, the value is
t. Otherwise, the value is an integer which indicates
how many leading characters agree, and which string is less. Its
absolute value is one plus the number of characters that agree at
the beginning of the two strings. The sign is negative if
string1 (or its specified portion) is less.
assoc,
except that key must be a string, and comparison is done
using compare-strings. Case differences are ignored in
this comparison.
assoc,
except that key must be a string, and comparison is done
using compare-strings. Case differences are
significant.
See also compare-buffer-substrings in section Comparing Text, for a way to compare
text in buffers. The function string-match, which
matches a regular expression against a string, can be used for a
kind of string comparison; see section Regular Expression Searching.
This section describes functions for conversions between
characters, strings and integers. format and
prin1-to-string (see section Output Functions) can also convert
Lisp objects into strings. read-from-string (see
section Input Functions) can
"convert" a string representation of a Lisp object into an object.
The functions string-make-multibyte and
string-make-unibyte convert the text representation of
a string (see section Converting Text
Representations).
See section Documentation, for
functions that produce textual descriptions of text characters and
general input events (single-key-description and
text-char-description). These functions are used
primarily for making help messages.
string is more general. See
section Creating Strings.
(string-to-char "ABC")
=> 65
(string-to-char "xyz")
=> 120
(string-to-char "")
=> 0
(string-to-char "\000")
=> 0
This function may be eliminated in the future if it does not seem useful enough to retain.
(number-to-string 256)
=> "256"
(number-to-string -23)
=> "-23"
(number-to-string -23.5)
=> "-23.5"
int-to-string is
a semi-obsolete alias for this function.
See also the function format in section Formatting Strings.
nil, integers are converted in that base. If
base is nil, then base ten is used.
Floating point conversion always uses base ten; we have not
implemented other radices for floating point numbers, because that
would be much more work and does not seem useful. The parsing skips spaces and tabs at the beginning of string, then reads as much of string as it can interpret as a number. (On some systems it ignores other whitespace at the beginning, not just spaces and tabs.) If the first character after the ignored whitespace is not a digit or a plus or minus sign, this function returns 0.
(string-to-number "256")
=> 256
(string-to-number "25 is a perfect square.")
=> 25
(string-to-number "X256")
=> 0
(string-to-number "-4.5")
=> -4.5
Here are some other functions that can convert to or from a string:
concatconcat can convert a vector or a list into a
string. See section Creating
Strings.
vconcatvconcat can convert a string into a vector. See
section Functions for Vectors.
appendappend can convert a string into a list. See
section Building Cons Cells and
Lists.
Formatting means constructing a string by substitution of computed values at various places in a constant string. This string controls how the other values are printed as well as where they appear; it is called a format string.
Formatting is often useful for computing messages to be
displayed. In fact, the functions message and
error provide the same formatting feature described
here; they differ from format only in how they use the
result of formatting.
A format specification is a sequence of
characters beginning with a `%'. Thus, if there is a
`%d' in string, the format
function replaces it with the printed representation of one of the
values to be formatted (one of the arguments objects).
For example:
(format "The value of fill-column is %d." fill-column)
=> "The value of fill-column is 72."
If string contains more than one format specification, the format specifications correspond with successive values from objects. Thus, the first format specification in string uses the first such value, the second format specification uses the second such value, and so on. Any extra format specifications (those for which there are no corresponding values) cause unpredictable behavior. Any extra values to be formatted are ignored.
Certain format specifications require values of particular types. If you supply a value that doesn't fit the requirements, an error is signaled.
Here is a table of valid format specifications:
princ, not prin1---see section Output Functions). Thus, strings are
represented by their contents alone, with no `"'
characters, and symbols appear without `\' characters.
If there is no corresponding object, the empty string is used.
prin1---see section Output
Functions). Thus, strings are enclosed in `"'
characters, and `\' characters appear where necessary
before special characters. If there is no corresponding object, the
empty string is used.
(format "%% %d" 30) returns "%
30".
Any other format character results in an `Invalid format operation' error.
Here are several examples:
(format "The name of this buffer is %s." (buffer-name))
=> "The name of this buffer is strings.texi."
(format "The buffer object prints as %s." (current-buffer))
=> "The buffer object prints as strings.texi."
(format "The octal value of %d is %o,
and the hex value is %x." 18 18 18)
=> "The octal value of 18 is 22,
and the hex value is 12."
All the specification characters allow an optional numeric prefix between the `%' and the character. The optional numeric prefix defines the minimum width for the object. If the printed representation of the object contains fewer characters than this, then it is padded. The padding is on the left if the prefix is positive (or starts with zero) and on the right if the prefix is negative. The padding character is normally a space, but if the numeric prefix starts with a zero, zeros are used for padding. Here are some examples of padding:
(format "%06d is padded on the left with zeros" 123)
=> "000123 is padded on the left with zeros"
(format "%-6d is padded on the right" 123)
=> "123 is padded on the right"
format never truncates an object's printed
representation, no matter what width you specify. Thus, you can use
a numeric prefix to specify a minimum spacing between columns with
no risk of losing information.
In the following three examples, `%7s' specifies a
minimum width of 7. In the first case, the string inserted in place
of `%7s' has only 3 letters, so 4 blank spaces are
inserted for padding. In the second case, the string
"specification" is 13 letters wide but is not
truncated. In the third case, the padding is on the right.
(format "The word `%7s' actually has %d letters in it."
"foo" (length "foo"))
=> "The word ` foo' actually has 3 letters in it."
(format "The word `%7s' actually has %d letters in it."
"specification" (length "specification"))
=> "The word `specification' actually has 13 letters in it."
(format "The word `%-7s' actually has %d letters in it."
"foo" (length "foo"))
=> "The word `foo ' actually has 3 letters in it."
The character case functions change the case of single characters or of the contents of strings. The functions normally convert only alphabetic characters (the letters `A' through `Z' and `a' through `z', as well as non-ASCII letters); other characters are not altered. (You can specify a different case conversion mapping by specifying a case table---see section The Case Table.)
These functions do not modify the strings that are passed to them as arguments.
The examples below use the characters `X' and `x' which have ASCII codes 88 and 120 respectively.
When the argument to downcase is a string, the
function creates and returns a new string in which each letter in
the argument that is upper case is converted to lower case. When
the argument to downcase is a character,
downcase returns the corresponding lower case
character. This value is an integer. If the original character is
lower case, or is not a letter, then the value equals the original
character.
(downcase "The cat in the hat")
=> "the cat in the hat"
(downcase ?X)
=> 120
When the argument to upcase is a string, the
function creates and returns a new string in which each letter in
the argument that is lower case is converted to upper case.
When the argument to upcase is a character,
upcase returns the corresponding upper case character.
This value is an integer. If the original character is upper case,
or is not a letter, then the value equals the original
character.
(upcase "The cat in the hat")
=> "THE CAT IN THE HAT"
(upcase ?x)
=> 88
The definition of a word is any sequence of consecutive characters that are assigned to the word constituent syntax class in the current syntax table (See section Table of Syntax Classes).
When the argument to capitalize is a character,
capitalize has the same result as
upcase.
(capitalize "The cat in the hat")
=> "The Cat In The Hat"
(capitalize "THE 77TH-HATTED CAT")
=> "The 77th-Hatted Cat"
(capitalize ?x)
=> 88
The definition of a word is any sequence of consecutive characters that are assigned to the word constituent syntax class in the current syntax table (See section Table of Syntax Classes).
(upcase-initials "The CAT in the hAt")
=> "The CAT In The HAt"
See section Comparison of Characters and Strings, for functions that compare strings; some of them ignore case differences, or can optionally ignore case differences.
You can customize case conversion by installing a special case table. A case table specifies the mapping between upper case and lower case letters. It affects both the case conversion functions for Lisp objects (see the previous section) and those that apply to text in the buffer (see section Case Changes). Each buffer has a case table; there is also a standard case table which is used to initialize the case table of new buffers.
A case table is a char-table (see section Char-Tables) whose subtype is
case-table. This char-table maps each character into
the corresponding lower case character. It has three extra slots,
which hold related tables:
In simple cases, all you need to specify is the mapping to lower-case; the three related tables will be calculated automatically from that one.
For some languages, upper and lower case letters are not in one-to-one correspondence. There may be two different lower case letters with the same upper case equivalent. In these cases, you need to specify the maps for both lower case and upper case.
The extra table canonicalize maps each character to a canonical equivalent; any two characters that are related by case-conversion have the same canonical equivalent character. For example, since `a' and `A' are related by case-conversion, they should have the same canonical equivalent character (which should be either `a' for both of them, or `A' for both of them).
The extra table equivalences is a map that cyclicly permutes each equivalence class (of characters with the same canonical equivalent). (For ordinary ASCII, this would map `a' into `A' and `A' into `a', and likewise for each set of equivalent characters.)
When you construct a case table, you can provide
nil for canonicalize; then Emacs fills in
this slot from the lower case and upper case mappings. You can also
provide nil for equivalences; then Emacs
fills in this slot from canonicalize. In a case table
that is actually in use, those components are non-nil.
Do not try to specify equivalences without also
specifying canonicalize.
Here are the functions for working with case tables:
nil if object is a valid case
table.
The following three functions are convenient subroutines for packages that define non-ASCII character sets. They modify the specified case table case-table; they also modify the standard syntax table. See section Syntax Tables. Normally you would use these functions to change the standard case table.