Function Dump/String Manipulation

< [[Function Dump]] | [[String Manipulation]]

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. 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).

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

Returns the internal numerical codes of the characters s[i], s[i+1], ···, 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.

Example

print(string.byte ("abc", 1, 3))

Will result in:
97 98 99

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.

Example

print(string.char (97, 98, 99, 100))

Will result in:
abcd

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. This function is commonly used in script obfuscation.

Example

function f () 
print "hello, world" 
end
s = string.dump (f)
assert (loadstring (s)) ()

Will result in:
hello, world [http://www.gammon.com.au/scripts/doc.php?lua=string.dump]

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 may 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.

Example

print(string.find ("blahblah", "bla"))

Will result in:
1 3

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.

Example

print (string.format ("To wield the %s you need to be level %i", "sword", 10))
Will result in:
To wield the sword you need to be level 10

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.

Example

for q in string.gmatch ("send money mom", "%a+") do
  print (q)
end

Will result in:
send 
money
mom

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 may 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"

Example

s = "I have some oranges. Oranges are red."

print(string.gsub(s,"Orange","Apple"))

Will result in:
I have some oranges. Apples are red. 1

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.

Example

print(string.len (""))
print(string.len ("a"))
print(string.len ("ab"))
print(string.len ("abc"))

Will result in:
0
1
2
3

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.

Example

print(string.lower ("Hi Mom!"))

Will result in:
hi mom!

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 may be negative.

Example

print (string.match ("I like pepperoni pizza", "pi..."))

Will result in:
pizza

string.rep (s, n)

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

Example

a=string.rep ("blah", 4) 
print(a)

Will result in:
blahblahblahblah

string.reverse (s)

Returns a string that is the string s reversed.

Example

print(string.reverse ("!moM ,olleH"))

Will result in:
Hello, Mom!

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.

Example

print(string.sub ("Hi Mom!", 1, 4))

Will result in:
Hi M

print(string.sub ("Hi Mom!", 1))

Will result in
Hi Mom!

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.

Example

print(string.upper ("Hi Mom!"))

Will result in:
HI MOM!

Patterns

Click here for info about patterns.