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Strings and Characters

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.

String and Character Basics

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.

The Predicates for Strings

For more information about general sequence and array predicates, see section Sequences, Arrays, and Vectors, and section Arrays.

Function: stringp object
This function returns t if object is a string, nil otherwise.

Function: char-or-string-p object
This function returns t if object is a string or a character (i.e., an integer), nil otherwise.

Creating Strings

The following functions create strings, either from scratch, or by putting strings together, or by taking them apart.

Function: make-string count character
This function returns a string made up of count repetitions of character. If count is negative, an error is signaled.

(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).

Function: string &rest characters
This returns a string containing the characters characters.

(string ?a ?b ?c)
     => "abc"

Function: substring string start &optional end
This function returns a new string which consists of those characters from string in the range from (and including) the character at the index start up to (but excluding) the character at the index end. The first character is at index zero.

(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.

Function: concat &rest sequences
This function returns a new string consisting of the characters in the arguments passed to it (along with their text properties, if any). The arguments may be strings, lists of numbers, or vectors of numbers; they are not themselves changed. If 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.

Function: split-string string separators
Split string into substrings in between matches for the regular expression separators. Each match for separators defines a splitting point; the substrings between the splitting points are made into a list, which is the value. If separators is 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" "!")

Modifying Strings

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:

Function: store-substring string idx obj
This function alters part of the contents of the string string, by storing obj starting at index idx. The argument obj may be either a character or a (smaller) string.

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.

Comparison of Characters and Strings

Function: char-equal character1 character2
This function returns 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

Function: string= string1 string2
This function returns 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.

Function: string-equal string1 string2
string-equal is another name for string=.

Function: string< string1 string2
This function compares two strings a character at a time. First it scans both the strings at once to find the first pair of corresponding characters that do not match. If the lesser character of those two is the character from string1, then string1 is less, and this function returns 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                   

Function: string-lessp string1 string2
string-lessp is another name for string<.

Function: compare-strings string1 start1 end1 string2 start2 end2 &optional ignore-case
This function compares a specified part of string1 with a specified part of string2. The specified part of string1 runs from index start1 up to index end1 (default, the end of the string). The specified part of string2 runs from index start2 up to index end2 (default, the end of the 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.

Function: assoc-ignore-case key alist
This function works like assoc, except that key must be a string, and comparison is done using compare-strings. Case differences are ignored in this comparison.

Function: assoc-ignore-representation key alist
This function works like 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.

Conversion of Characters and Strings

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.

Function: char-to-string character
This function returns a new string containing one character, character. This function is semi-obsolete because the function string is more general. See section Creating Strings.

Function: string-to-char string
This function returns the first character in string. If the string is empty, the function returns 0. The value is also 0 when the first character of string is the null character, ASCII code 0.

(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.

Function: number-to-string number
This function returns a string consisting of the printed representation of number, which may be an integer or a floating point number. The value starts with a sign if the argument is negative.

(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.

Function: string-to-number string &optional base
This function returns the numeric value of the characters in string. If base is non-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

string-to-int is an obsolete alias for this function.

Here are some other functions that can convert to or from a string:

concat
concat can convert a vector or a list into a string. See section Creating Strings.
vconcat
vconcat can convert a string into a vector. See section Functions for Vectors.
append
append can convert a string into a list. See section Building Cons Cells and Lists.

Formatting Strings

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.

Function: format string &rest objects
This function returns a new string that is made by copying string and then replacing any format specification in the copy with encodings of the corresponding objects. The arguments objects are the computed values to be formatted.

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:

`%s'
Replace the specification with the printed representation of the object, made without quoting (that is, using 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.
`%S'
Replace the specification with the printed representation of the object, made with quoting (that is, using 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.
`%o'
Replace the specification with the base-eight representation of an integer.
`%d'
Replace the specification with the base-ten representation of an integer.
`%x'
Replace the specification with the base-sixteen representation of an integer.
`%c'
Replace the specification with the character which is the value given.
`%e'
Replace the specification with the exponential notation for a floating point number.
`%f'
Replace the specification with the decimal-point notation for a floating point number.
`%g'
Replace the specification with notation for a floating point number, using either exponential notation or decimal-point notation, whichever is shorter.
`%%'
A single `%' is placed in the string. This format specification is unusual in that it does not use a value. For example, (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."  

Case Conversion in Lisp

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.

Function: downcase string-or-char
This function converts a character or a string to lower case.

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

Function: upcase string-or-char
This function converts a character or a string to upper case.

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

Function: capitalize string-or-char
This function capitalizes strings or characters. If string-or-char is a string, the function creates and returns a new string, whose contents are a copy of string-or-char in which each word has been capitalized. This means that the first character of each word is converted to upper case, and the rest are converted to lower case.

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

Function: upcase-initials string
This function capitalizes the initials of the words in string. without altering any letters other than the initials. It returns a new string whose contents are a copy of string, in which each word has been converted to upper case.

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.

The Case Table

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:

upcase
The upcase table maps each character into the corresponding upper case character.
canonicalize
The canonicalize table maps all of a set of case-related characters into a particular member of that set.
equivalences
The equivalences table maps each one of a set of case-related characters into the next character in that set.

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:

Function: case-table-p object
This predicate returns non-nil if object is a valid case table.

Function: set-standard-case-table table
This function makes table the standard case table, so that it will be used in any buffers created subsequently.

Function: standard-case-table
This returns the standard case table.

Function: current-case-table
This function returns the current buffer's case table.

Function: set-case-table table
This sets the current buffer's case table to 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.

Function: set-case-syntax-pair uc lc case-table
This function specifies a pair of corresponding letters, one upper case and one lower case.

Function: set-case-syntax-delims l r case-table
This function makes characters l and r a matching pair of case-invariant delimiters.

Function: set-case-syntax char syntax case-table
This function makes char case-invariant, with syntax syntax.

Command: describe-buffer-case-table
This command displays a description of the contents of the current buffer's case table.


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