Next: String Conversion, Previous: Modifying Strings, Up: Strings and Characters [Contents][Index]

### 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

  For technical reasons, a unibyte and a multibyte string are `equal` if and only if they contain the same sequence of character codes and all these codes are either in the range 0 through 127 (ASCII) or 160 through 255 (`eight-bit-graphic`). However, when a unibyte string is converted to a multibyte string, all characters with codes in the range 160 through 255 are converted to characters with higher codes, whereas ASCII characters remain unchanged. Thus, a unibyte string and its conversion to multibyte are only `equal` if the string is all ASCII. Character codes 160 through 255 are not entirely proper in multibyte text, even though they can occur. As a consequence, the situation where a unibyte and a multibyte string are `equal` without both being all ASCII is a technical oddity that very few Emacs Lisp programmers ever get confronted with. See Text Representations.

Function: string-equal string1 string2

  `string-equal` is another name for `string=`.

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

  This function returns `t` if string1 and string2 are equal with respect to collation rules. A collation rule is not only determined by the lexicographic order of the characters contained in string1 and string2, but also further rules about relations between these characters. Usually, it is defined by the locale environment Emacs is running with.

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

      (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 converted to lower-case before comparing them.

  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-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< 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. A collation order is not only determined by the lexicographic order of the characters contained in string1 and string2, but also further rules about relations between these characters. Usually, it is defined by the locale environment Emacs is running with.

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

      (sort '("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 '("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 converted to lower-case before comparing them.

  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-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: 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 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: 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 Regexp Search.

Next: String Conversion, Previous: Modifying Strings, Up: Strings and Characters [Contents][Index]