how to extracted updated string after calling string.gsub in lua? - string

Question

Problem Description:
HI there. I'm trying to figure out how to use the lua function "string.gsub". I've been reading the manual which says:
This is a very powerful function and can be used in multiple ways.
Used simply it can replace all instances of the pattern provided with
the replacement. A pair of values is returned, the modified string and
the number of substitutions made. The optional fourth argument n can
be used to limit the number of substitutions made:
> = string.gsub("Hello banana", "banana", "Lua user")
Hello Lua user 1
> = string.gsub("banana", "a", "A", 2) -- limit substitutions made to 2
bAnAna 2
Question
When it says that a pair of values is returned; how do I get the new string value?
Code
local email_filename = "/var/log/test.txt"
local email_contents_file_exists = function(filename)
file = io.open(filename, "r")
if file == nil then
return false
else
file.close(file)
return true
end
end
local read_email_contents_file = function()
print('inside the function')
if not email_contents_file_exists(email_filename) then
return false
end
local f = io.open(email_filename, "rb")
local content = f:read("*all")
f:close()
print(content)
--content = string.gsub(content, '[username]', 'myusername')
--local tmp {}
--tmp = string.gsub(content, '[username]', 'myusername')
print(string.gsub(content, '[username]', 'myusername'))
return content
end
local test = read_email_contents_file()
What I've Tried So Far:
I've tried just printing the results, as you see above. That returns a bunch of garbled text. Tried saving to original string and I've also tried saving the results to an array (local tmp = {})
Any suggestions?

Answer 1

> = string.gsub('banana', 'a', 'A', 2)
bAnAna 2
> = (string.gsub('banana', 'a', 'A', 2))
bAnAna

Answer 2

You were going pretty good with reading the Lua users wiki.
In Lua, when you a function returns more than one value, you can access them all as follows
function sth()
return 1, "hi", false
end
x, y, z, a, b, c = sth() -- x = 1; y = "hi" and z = false(boolean); a = b = c = nil
Now, coming back to string.gsub function. It returns two values. The first being the processed string and the second being the number of time gsub performed itself on the input string.
So, to get the new string value, something like this would be best:
local tempString = string.gsub(content, '[username]', 'myusername')
OR
local tempString = content:gsub( '[username]', 'myusername' )
Ofcourse, here, you need to be aware about the various patterns used in Lua which are mentioned in the Programming in Lua book.

Answer 3

You need to escape [ and ] because they are magic characters in Lua patterns.