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string - string manipulation functions (standard library)

OVERVIEW

This library provides generic functions for string manipulation, such as finding and extracting substrings, and pattern matching. When indexing a string in Lua, the first character is at position 1 (not at 0, as in C). Indices are allowed to be negative and are interpreted as indexing backwards, from the end of the string. Thus, the last character is at position -1, and so on.

The string library provides all its functions inside the table string. It also sets a metatable for strings where the __index field points to the string table. Therefore, you can use the string functions in object-oriented style. For instance, string.byte(s, i) can be written as s:byte(i).

The string library assumes one-byte character encodings.

INTERFACE

string.byte()
string.byte (s [, i [, j]])

Returns the internal numerical codes of the characters s[i], s[i+1], middotmiddotmiddot, s[j]. The default value for i is 1; the default value for j is i.

Note that numerical codes are not necessarily portable across platforms.

string.char()
string.char (...)

Receives zero or more integers. Returns a string with length equal to the number of arguments, in which each character has the internal numerical code equal to its corresponding argument.

Note that numerical codes are not necessarily portable across platforms.

string.dump()
string.dump (function)

Returns a string containing a binary representation of the given function, so that a later loadstring on this string returns a copy of the function. function must be a Lua function without upvalues.

string.find()
string.find (s, pattern [, init [, plain]])

Looks for the first match of pattern in the string s. If it finds a match, then find returns the indices of s where this occurrence starts and ends; otherwise, it returns nil. A third, optional numerical argument init specifies where to start the search; its default value is 1 and can be negative. A value of true as a fourth, optional argument plain turns off the pattern matching facilities, so the function does a plain “find substring” operation, with no characters in pattern being considered “magic”. Note that if plain is given, then init must be given as well.

If the pattern has captures, then in a successful match the captured values are also returned, after the two indices.

string.format()
string.format (formatstring, ...)

Returns a formatted version of its variable number of arguments following the description given in its first argument (which must be a string). The format string follows the same rules as the printf family of standard C functions. The only differences are that the options/modifiers \*, l, L, n, p, and h are not supported and that there is an extra option, q. The q option formats a string in a form suitable to be safely read back by the Lua interpreter: the string is written between double quotes, and all double quotes, newlines, embedded zeros, and backslashes in the string are correctly escaped when written. For instance, the call

string.format('%q', 'a string with "quotes" and \n new line')

will produce the string:

"a string with \"quotes\" and \
 new line"

The options c, d, E, e, f, g, G, i, o, u, X, and x all expect a number as argument, whereas q and s expect a string.

This function does not accept string values containing embedded zeros, except as arguments to the q option.

string.gmatch()
string.gmatch (s, pattern)

Returns an iterator function that, each time it is called, returns the next captures from pattern over string s. If pattern specifies no captures, then the whole match is produced in each call.

As an example, the following loop

s = "hello world from Lua"
for w in string.gmatch(s, "%a+") do
  print(w)
end

will iterate over all the words from string s, printing one per line. The next example collects all pairs key=value from the given string into a table:

t = {}
s = "from=world, to=Lua"
for k, v in string.gmatch(s, "(%w+)=(%w+)") do
  t[k] = v
end

For this function, a ‘^‘ at the start of a pattern does not work as an anchor, as this would prevent the iteration.

string.gsub()
string.gsub (s, pattern, repl [, n])

Returns a copy of s in which all (or the first n, if given) occurrences of the pattern have been replaced by a replacement string specified by repl, which can be a string, a table, or a function. gsub also returns, as its second value, the total number of matches that occurred.

If repl is a string, then its value is used for replacement. The character % works as an escape character: any sequence in repl of the form %``*n*, with *n* between 1 and 9, stands for the value of the *n*-th captured substring (see below). The sequence ``%0 stands for the whole match. The sequence %% stands for a single %.

If repl is a table, then the table is queried for every match, using the first capture as the key; if the pattern specifies no captures, then the whole match is used as the key.

If repl is a function, then this function is called every time a match occurs, with all captured substrings passed as arguments, in order; if the pattern specifies no captures, then the whole match is passed as a sole argument.

If the value returned by the table query or by the function call is a string or a number, then it is used as the replacement string; otherwise, if it is false or nil, then there is no replacement (that is, the original match is kept in the string).

Here are some examples:

x = string.gsub("hello world", "(%w+)", "%1 %1")
--> x="hello hello world world"

x = string.gsub("hello world", "%w+", "%0 %0", 1)
--> x="hello hello world"

x = string.gsub("hello world from Lua", "(%w+)%s*(%w+)", "%2 %1")
--> x="world hello Lua from"

x = string.gsub("home = $HOME, user = $USER", "%$(%w+)", os.getenv)
--> x="home = /home/roberto, user = roberto"

x = string.gsub("4+5 = $return 4+5$", "%$(.-)%$", function (s)
      return loadstring(s)()
    end)
--> x="4+5 = 9"

local t = {name="lua", version="5.1"}
x = string.gsub("$name-$version.tar.gz", "%$(%w+)", t)
--> x="lua-5.1.tar.gz"
string.len()
string.len (s)

Receives a string and returns its length. The empty string "" has length 0. Embedded zeros are counted, so "a\000bc\000" has length 5.

string.lower()
string.lower (s)

Receives a string and returns a copy of this string with all uppercase letters changed to lowercase. All other characters are left unchanged. The definition of what an uppercase letter is depends on the current locale.

string.match()
string.match (s, pattern [, init])

Looks for the first match of pattern in the string s. If it finds one, then match returns the captures from the pattern; otherwise it returns nil. If pattern specifies no captures, then the whole match is returned. A third, optional numerical argument init specifies where to start the search; its default value is 1 and can be negative.

string.rep()
string.rep (s, n)

Returns a string that is the concatenation of n copies of the string s.

string.reverse()
string.reverse (s)

Returns a string that is the string s reversed.

string.sub()
string.sub (s, i [, j])

Returns the substring of s that starts at i and continues until j; i and j can be negative. If j is absent, then it is assumed to be equal to -1 (which is the same as the string length). In particular, the call string.sub(s,1,j) returns a prefix of s with length j, and string.sub(s,-i) returns a suffix of s with length i.

string.upper()
string.upper (s)

Receives a string and returns a copy of this string with all lowercase letters changed to uppercase. All other characters are left unchanged. The definition of what a lowercase letter is depends on the current locale.

Patterns

Character Class

A character class is used to represent a set of characters. The following combinations are allowed in describing a character class:

  • x (where x is not one of the magic characters ^$()%.[]\*+-?) represents the character x itself.
  • . (a dot) represents all characters.
  • %a represents all letters.
  • %c represents all control characters.
  • %d represents all digits.
  • %l represents all lowercase letters.
  • %p represents all punctuation characters.
  • %s represents all space characters.
  • %u represents all uppercase letters.
  • %w represents all alphanumeric characters.
  • %x represents all hexadecimal digits.
  • %z represents the character with representation 0.
  • %x (where x is any non-alphanumeric character) represents the character x. This is the standard way to escape the magic characters. Any punctuation character (even the non magic) can be preceded by a ‘%‘ when used to represent itself in a pattern.
  • [set] represents the class which is the union of all characters in set. A range of characters can be specified by separating the end characters of the range with a ‘-‘. All classes %``*x* described above can also be used as components in *set*.  All other characters in *set* represent themselves.  For example, ``[%w_] (or [_%w]) represents all alphanumeric characters plus the underscore, [0-7] represents the octal digits, and [0-7%l%-] represents the octal digits plus the lowercase letters plus the ‘-‘ character. The interaction between ranges and classes is not defined. Therefore, patterns like [%a-z] or [a-%%] have no meaning.
  • [^set] represents the complement of set, where set is interpreted as above.

For all classes represented by single letters (%a, %c, etc.), the corresponding uppercase letter represents the complement of the class. For instance, %S represents all non-space characters.

The definitions of letter, space, and other character groups depend on the current locale. In particular, the class [a-z] may not be equivalent to %l.

Pattern Item:

A pattern item can be

a single character class, which matches any single character in the class;

a single character class followed by ‘\*‘, which matches 0 or more repetitions of characters in the class. These repetition items will always match the longest possible sequence;

a single character class followed by ‘+‘, which matches 1 or more repetitions of characters in the class. These repetition items will always match the longest possible sequence;

a single character class followed by ‘-‘, which also matches 0 or more repetitions of characters in the class. Unlike ‘\*‘, these repetition items will always match the shortest possible sequence;

a single character class followed by ‘?‘, which matches 0 or 1 occurrence of a character in the class;

%*n*, for n between 1 and 9; such item matches a substring equal to the n-th captured string (see below);

%b*xy*, where x and y are two distinct characters; such item matches strings that start with x, end with y, and where the x and y are balanced. This means that, if one reads the string from left to right, counting +1 for an x and -1 for a y, the ending y is the first y where the count reaches 0. For instance, the item %b() matches expressions with balanced parentheses.

Pattern:

A pattern is a sequence of pattern items.

A ‘^‘ at the beginning of a pattern anchors the match at the beginning of the subject string.

A ‘$‘ at the end of a pattern anchors the match at the end of the subject string. At other positions,

^‘ and ‘$‘ have no special meaning and represent themselves.

Captures

A pattern can contain sub-patterns enclosed in parentheses; they describe captures. When a match succeeds, the substrings of the subject string that match captures are stored (captured) for future use. Captures are numbered according to their left parentheses. For instance, in the pattern "(a\*(.)%w(%s\*))", the part of the string matching "a\*(.)%w(%s\*)" is stored as the first capture (and therefore has number 1); the character matching “.” is captured with number 2, and the part matching “%s\*” has number 3.

As a special case, the empty capture () captures the current string position (a number). For instance, if we apply the pattern "()aa()" on the string "flaaap", there will be two captures: 3 and 5.

A pattern cannot contain embedded zeros. Use %z instead.


This document is from Lua version 5.1.5. Copyright (c) 2006-2012 Lua.org, PUC-Rio. Freely available under the terms of the Lua license.

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