Strings Tutorial

wiki

Quotes

Strings are introduced in section 2.1 of the Reference Manual [1]. Strings can be defined using single quotes, double quotes, or double square brackets.

> = "hello"

hello

> = 'hello'

hello

> = [[hello]]

hello

Why so many ways to make a string? It allows you to enclose one type of quotes in the other. e.g.,

> = 'hello "Lua user"'

hello "Lua user"

> = "Its [[content]] hasn't got a substring."

Its [[content]] hasn't got a substring.

> = [[Let's have more "strings" please.]]

Let's have more "strings" please.

Double bracketed strings also have a few other special properties, discussed below.

Escape sequences

Lua can also handle C-like escape sequences. There are more details in the Reference Manual, section 2.1 [1].

> = "hello \"Lua user\""

hello "Lua user"

> = 'hello\nNew line\tTab'

hello

New line        Tab

Escape sequences are not recognized when using double brackets, so:

> = [[hello\nNew line\tTab]]



hello\nNew line\tTab

Multiline quotes

Double square brackets can be used to enclose literal strings which traverse several lines. e.g.,

> = [[Multiple lines of text

>> can be enclosed in double square

>> brackets.]]

Multiple lines of text

can be enclosed in double square

brackets.

Nesting quotes

Double square brackets allow nesting, but they require one or more = inserted in the outer-most brackets to distinguish them. It doesn't matter how many = are inserted, as long as the number is the same in the beginning and ending brackets.

> = [[one [[two]] one]]        -- bad

stdin:1: nesting of [[...]] is deprecated near '['

> = [=[one [[two]] one]=]      -- ok

one [[two]] one

> = [===[one [[two]] one]===]  -- ok too

one [[two]] one

> = [=[one [ [==[ one]=]       -- ok. nothing special about the inner content.

one [ [==[ one

Concatenation

Strings can be joined together using the concatenation operator "..". e.g.,

> = "hello" .. " Lua user"

hello Lua user

> who = "Lua user"

> = "hello "..who

hello Lua user

> = "Green bottles: "..10

Green bottles: 10

> = type("Green bottles: "..10)

string

Doing a large number of concatenation operations may be slow because each concatenation may allocate a new string in memory. The following three examples give the same result except the first may be much slower:

-- slow

local s = ''

for i=1,10000 do s = s .. math.random() .. ',' end

io.stdout:write(s)



-- fast

for i=1,10000 do io.stdout:write(tostring(math.random()), ',') end



-- fast, but uses more memory

local t = {}

for i=1,10000 do t[i] = tostring(math.random()) end

io.stdout:write(table.concat(t,','), ',') 

The string library

Lua supplies a range of useful functions for processing and manipulating strings in its standard library. More details are supplied in the StringLibraryTutorial. Below are a few examples of usage of the string library.

> = string.byte("ABCDE", 2) -- return the ASCII value of the second character

66

> = string.char(65,66,67,68,69) -- return a string constructed from ASCII values

ABCDE

> = string.find("hello Lua user", "Lua") -- find substring "Lua"

7       9

> = string.find("hello Lua user", "l+") -- find one or more occurrences of "l"

3       4

> = string.format("%.7f", math.pi) -- format a number

3.1415927

> = string.format("%8s", "Lua") -- format a string

     Lua

Coercion

Lua performs automatic conversion of numbers to strings and vice versa where it is appropriate. This is called coercion.

> = "This is Lua version " .. 5.1 .. " we are using."

This is Lua version 5.1 we are using.

> = "Pi = " .. math.pi

Pi = 3.1415926535898

> = "Pi = " .. 3.1415927

Pi = 3.1415927

> = string.format("%.3f", 5.1)

5.100

> = "Lua version " .. string.format("%.1f", 5.1)

Lua version 5.1