5.3 – String Manipulation

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.

string.byte (s [, i])

Returns the internal numerical code of the i-th character of s, or nil if the index is out of range. If i is absent, then it is assumed to be 1. i may be negative.

Note that numerical codes are not necessarily portable across platforms.

string.char (i1, i2, …)

Receives 0 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 correspondent argument.

Note that numerical codes are not necessarily portable across platforms.

string.dump (function)

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

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

Looks for the first match of pattern in the string s. If it finds one, then find returns the indices of s where this occurrence starts and ends; otherwise, it returns nil. If the pattern specifies captures (see string.gsub below), the captured strings are returned as extra results. A third, optional numerical argument init specifies where to start the search; it may be negative and its default value is 1. 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 too.

string.len (s)

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

string.lower (s)

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

string.rep (s, n)

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

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

Returns the substring of s that starts at i and continues until j; i and j may 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 (s)

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

string.format (formatstring, e1, e2, …)

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 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, 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. The * modifier can be simulated by building the appropriate format string. For example, "%*g" can be simulated with "%"..width.."g".

String values to be formatted with %s cannot contain embedded zeros.

string.gfind (s, pat)

Returns an iterator function that, each time it is called, returns the next captures from pattern pat over string s.

If pat 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.gfind(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.gfind(s, "(%w+)=(%w+)") do
    t[k] = v
  end

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

Returns a copy of s in which all occurrences of the pattern pat have been replaced by a replacement string specified by repl. gsub also returns, as a second value, the total number of substitutions made.

If repl is a string, then its value is used for replacement. 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).

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 this function is a string, then it is used as the replacement string; otherwise, the replacement string is the empty string.

The optional last parameter n limits the maximum number of substitutions to occur. For instance, when n is 1 only the first occurrence of pat is replaced.

Here are some examples:

   x = string.gsub("hello world", "(%w+)", "%1 %1")
   --> x="hello hello world world"
 
   x = string.gsub("hello world", "(%w+)", "%1 %1", 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="5.0"}
   x = string.gsub("$name_$version.tar.gz", "%$(%w+)", function (v)
         return t[v]
       end)
   --> x="lua_5.0.tar.gz"

Patterns

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 may be specified by separating the end characters of the range with a `-´. All classes %x described above may 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. The second form should be preferred for portability.

A pattern item may 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.

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.

A pattern may 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.