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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). Parameters written in brackets are optional.

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

Returns ascii values of the characters s[i], s[i+1], all the way until 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 ("d"))
print(string.byte ("abc", 1, 3))

Will result in:
100
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 is the ascii representation equal to its corresponding number.

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 function, so that a later loadstring on this string returns a copy of function. string.dump 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, substring [, init [, plain]])

Looks for the first match of substring in string s. substring may be a pattern. If a match is found, then string.find returns the locations (1, 3, etc.) of s where this occurrence starts and ends; otherwise, it returns nil. A third, optional numerical argument, init, specifies where to start the search in the string; 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" (see patterns). Note that if plain is given, then init must be given as well.

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

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). Here is a link to string formatting.

string.len (s)

Receives s (a string) and returns its length. The empty string "" has length 0. Embedded zeros (the null terminator) 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 s (a string) and returns a copy of s with all uppercase letters changed to lowercase. All other characters are left unchanged.

Example

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

Will result in:
hi mom!

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

Looks for the first match of string pattern in string s. If it finds one, then match returns the captures from pattern; otherwise, it returns nil. If pattern specifies no captures (see pattern 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. A series of dots (...) represent unknown characters; they will match anything.

Example

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

Will result in:
pizz

In the above example, the pi.. represented a string that started with pi and the next two characters; hence the two dots. Note that this overwrites the character class . (see patterns).

string.rep (s, n)

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

Example

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

Will result in:
Blarg, Blarg, Blarg, Blarg, 

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 the length of s. In particular, the call string.sub(s,1,j) returns s until as many characters as 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!", 2))

Will result in
i 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.

Example

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

Will result in:
HI MOM!

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 (see 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 index=value from the given string into a table:

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

For this function, a '^' at the start of pattern does not work as an anchor, as this would prevent the iteration. Here are some examples:

Example

local t={}
for matchedValue in string.gmatch("alphabet and numbers 1234-4321", "a") do -- Search for a in that string
    table.insert(t,matchedValue)
end
print(#t.." a's were found!")
Output:
3 a's were found!

for x in string.gmatch("Numbers", "b") do -- Search for b in Numbers
print(x)
end

Output:
b

Alright, that was searching for plain text. Here are some examples with patterns:

Example

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

Output:
send 
money
mom

for value in string.gmatch("Once upon a time, there were 34 fairies, 20 of which were smart.", "%d+") do -- Find numbers
    print("At least "..value.." of the fairies are magical.")
end

Output:
At least 34 of the fairies are magical.
At least 20 of the fairies are magical.

for q in string.gmatch("My name is merlin11188", "(%a+)%d+") do
    print("The letter part of my name is "..q)
end

Output:
The letter part of my name is merlin

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

Returns a copy of s in which all (or the first n, if given) occurrences of pattern have been replaced by replacement string repl, which may be a string, a table, or a function. string.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 pattern captures). The sequence %0 stands for the whole match.

If repl is a table, then the table is queried for every match, using the first capture as the key; if 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 pattern specifies no captures, then the whole match is passed as the 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 using plain character replacement:

Example

x = string.gsub("hello world", "hello", "goodbye")
--> x="goodbye world"
     
x = string.gsub("Bonjour!", "Bonjour", "Hello")
--> x="Hello!"

x = string.gsub("Avada Kedavra", "Avada Kedavra", "Abracadabra")
--> x="Abracadabra"

x = string.gsub("I like to munch numbers 1234", "1234", "and words.")
--> x="I like to munch numbers and words.")

Those were all examples using plain character replacements. Here are some using patterns:

Example

x = string.gsub("I found an imaginary hex number: F243BC234AD234Ei", "%x+i", "Yeah right. That's not real.")
--> x="I found an imaginary hex number: Yeah right. That's not real."
     
x = string.gsub("home = $HOME, user = $USER", "%$(%w+)", {USER="Merlin", HOME="AWESOME"})
--> x="home = AWESOME, user = Merlin"
     
x = string.gsub("4+5 = $return 4+5$", "%$(.-)%$", function (s)
    return loadstring(s)()
end)
--> x="4+5 = 9"

x = string.gsub("hello world from Lua", "(%w+)%s*(%w+)", "%2 %1")
--> x="world hello Lua from"
     
local t = {name="lua", version="5.1"}
x = string.gsub("$name-$version.tar.gz", "%$(%w+)", t)
--> x="lua-5.1.tar.gz"

See Also

---

Patterns

A Beginner's Guide to String Patterns