How can Lua retrieve a specific character from a string position?

In Lua, you can use string indexing to retrieve a specific character at a certain position. String indexing starts at 1, indicating the first character of the string. Below is an example code for retrieving a character at a specified position in a string:

local str = "Hello, Lua!"

-- 获取第一个字符
local firstChar = str:sub(1, 1)
print(firstChar)  -- 输出 "H"

-- 获取第五个字符
local fifthChar = str:sub(5, 5)
print(fifthChar)  -- 输出 "o"

-- 获取最后一个字符
local lastChar = str:sub(-1)
print(lastChar)  -- 输出 "!"

-- 获取倒数第三个字符
local thirdLastChar = str:sub(-3, -3)
print(thirdLastChar)  -- 输出 "u"

In the above example, the str:sub(index, index) function is used to retrieve a specific character at a given position. Positive indices can be used to get characters from left to right, while negative indices can be used to get characters from right to left.