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4.5 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. Symbols are also allowed as arguments, in which case the symbol names are used. Case is always significant, regardless of case-fold-search.

This function is equivalent to equal for comparing two strings (see Equality Predicates). In particular, the text properties of the two strings are ignored; use equal-including-properties if you need to distinguish between strings that differ only in their text properties. However, unlike equal, if either argument is not a string or symbol, string= signals an error.

(string= "abc" "abc")
     ⇒ t
(string= "abc" "ABC")
     ⇒ nil
(string= "ab" "ABC")
     ⇒ nil

A unibyte and a multibyte string are equal in the sense of string= if and only if they contain the same sequence of character codes all being in the range 0–127 (ASCII). See Text Representations.

Function: string-equal string1 string2 ¶

string-equal is another name for string=.

Function: string-equal-ignore-case string1 string2 ¶

string-equal-ignore-case compares strings ignoring case differences, like char-equal when case-fold-search is t.

Function: string-collate-equalp string1 string2 &optional locale ignore-case ¶

This function returns t if string1 and string2 are equal with respect to the collation rules of the specified locale, which defaults to your current system locale. A collation rule is not only determined by the lexicographic order of the characters contained in string1 and string2, but also by further rules about relations between these characters. Usually, it is defined by the locale environment with which Emacs is running and by the Standard C library against which Emacs was linked³.

For example, characters with different code points but the same meaning, like different grave accent Unicode characters, might, in some locales, be considered as equal:

(string-collate-equalp (string ?\uFF40) (string ?\u1FEF))
     ⇒ t

The optional argument locale, a string, overrides the setting of your current locale identifier for collation. The value is system dependent; a locale "en_US.UTF-8" is applicable on POSIX systems, while it would be, e.g., "enu_USA.1252" on MS-Windows systems.

If ignore-case is non-nil, characters are compared case-insensitively, by converting them to lower-case. However, if the underlying system library doesn’t provide locale-specific collation rules, this function falls back to string-equal, in which case the ignore-case argument is ignored, and the comparison will always be case-sensitive.

To emulate Unicode-compliant collation on MS-Windows systems, bind w32-collate-ignore-punctuation to a non-nil value, since the codeset part of the locale cannot be "UTF-8" on MS-Windows.

If your system does not support a locale environment, this function behaves like string-equal.

Do not use this function to compare file names for equality, as filesystems generally don’t honor linguistic equivalence of strings that collation implements.

Function: string< string1 string2 ¶

This function compares two strings a character at a time. It scans both the strings at the same time to find the first pair of corresponding characters that do not match. If the lesser character of these 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 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

Symbols are also allowed as arguments, in which case their print names are compared.

Function: string-lessp string1 string2 ¶

string-lessp is another name for string<.

Function: string-greaterp string1 string2 ¶

This function returns the result of comparing string1 and string2 in the opposite order, i.e., it is equivalent to calling (string-lessp string2 string1).

Function: string-collate-lessp string1 string2 &optional locale ignore-case ¶

This function returns t if string1 is less than string2 in collation order of the specified locale, which defaults to your current system locale. A collation order is not only determined by the lexicographic order of the characters contained in string1 and string2, but also by further rules about relations between these characters. Usually, it is defined by the locale environment with which Emacs is running, and by the Standard C library against which Emacs was linked.

For example, punctuation and whitespace characters might be ignored for sorting (see Sequences):

(sort (list "11" "12" "1 1" "1 2" "1.1" "1.2") 'string-collate-lessp)
     ⇒ ("11" "1 1" "1.1" "12" "1 2" "1.2")

This behavior is system-dependent; e.g., punctuation and whitespace are never ignored on Cygwin, regardless of locale.

The optional argument locale, a string, overrides the setting of your current locale identifier for collation. The value is system dependent; a locale "en_US.UTF-8" is applicable on POSIX systems, while it would be, e.g., "enu_USA.1252" on MS-Windows systems. The locale value of "POSIX" or "C" lets string-collate-lessp behave like string-lessp:

(sort (list "11" "12" "1 1" "1 2" "1.1" "1.2")
      (lambda (s1 s2) (string-collate-lessp s1 s2 "POSIX")))
     ⇒ ("1 1" "1 2" "1.1" "1.2" "11" "12")

If ignore-case is non-nil, characters are compared case-insensitively, by converting them to lower-case. However, if the underlying system library doesn’t provide locale-specific collation rules, this function falls back to string-lessp, in which case the ignore-case argument is ignored, and the comparison will always be case-sensitive.

To emulate Unicode-compliant collation on MS-Windows systems, bind w32-collate-ignore-punctuation to a non-nil value, since the codeset part of the locale cannot be "UTF-8" on MS-Windows.

If your system does not support a locale environment, this function behaves like string-lessp.

Function: string-version-lessp string1 string2 ¶

This function compares strings lexicographically, except it treats sequences of numerical characters as if they comprised a base-ten number, and then compares the numbers. So ‘foo2.png’ is “smaller” than ‘foo12.png’ according to this predicate, even if ‘12’ is lexicographically “smaller” than ‘2’.

Function: string-prefix-p string1 string2 &optional ignore-case ¶

This function returns non-nil if string1 is a prefix of string2; i.e., if string2 starts with string1. If the optional argument ignore-case is non-nil, the comparison ignores case differences.

Function: string-suffix-p suffix string &optional ignore-case ¶

This function returns non-nil if suffix is a suffix of string; i.e., if string ends with suffix. If the optional argument ignore-case is non-nil, the comparison ignores case differences.

Function: string-search needle haystack &optional start-pos ¶

Return the position of the first instance of needle in haystack, both of which are strings. If start-pos is non-nil, start searching from that position in haystack. Return nil if no match was found. This function only considers the characters in the strings when doing the comparison; text properties are ignored. Matching is always case-sensitive.

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 (inclusive) up to index end1 (exclusive); nil for start1 means the start of the string, while nil for end1 means the length of the string. Likewise, the specified part of string2 runs from index start2 up to index end2.

The strings are compared by the numeric values of their characters. For instance, str1 is considered less than str2 if its first differing character has a smaller numeric value. If ignore-case is non-nil, characters are converted to upper-case, using the current buffer’s case-table (see The Case Table), before comparing them. Unibyte strings are converted to multibyte for comparison (see Text Representations), so that a unibyte string and its conversion to multibyte are always regarded as equal.

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: string-distance string1 string2 &optional bytecompare ¶

This function returns the Levenshtein distance between the source string string1 and the target string string2. The Levenshtein distance is the number of single-character changes—deletions, insertions, or replacements—required to transform the source string into the target string; it is one possible definition of the edit distance between strings.

Letter-case of the strings is significant for the computed distance, but their text properties are ignored. If the optional argument bytecompare is non-nil, the function calculates the distance in terms of bytes instead of characters. The byte-wise comparison uses the internal Emacs representation of characters, so it will produce inaccurate results for multibyte strings that include raw bytes (see Text Representations); make the strings unibyte by encoding them (see Explicit Encoding and Decoding) if you need accurate results with raw bytes.

Function: assoc-string key alist &optional case-fold ¶

This function works like assoc, except that key must be a string or symbol, and comparison is done using compare-strings. Symbols are converted to strings before testing. If case-fold is non-nil, key and the elements of alist are converted to upper-case before comparison. Unlike assoc, this function can also match elements of the alist that are strings or symbols rather than conses. In particular, alist can be a list of strings or symbols rather than an actual alist. See Association Lists.

See also the function compare-buffer-substrings in 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 Regular Expression Searching.