:gsub not working as expected in lua script

I am writing a lua script & want to perform an operation like-

input_string = "smart app"
output_string = "app"

input_string = "smart" or input_string = "smart "
output_string = nil (null)

I am using the below lua snippet to replace "smart " with empty string so that it solves both the cases.

output_string = input_string:gsub("smart ", "")

However I get below responses for each case respectively.

input_string = "smart app"
output_string = "app"

input_string = "smart" or imput_string = "smart "
output_string = "smart"

Why does it not work for case 2? Where am I making mistake? Please suggest.

P.s. I do not want to write separate logic to handle both cases, ideally my snippet should solve the case in one line.

UPDATE QUESTION:

in case

> input_string = "smr" or "smr "

I mean any other word, then also I expect output_string = nil

You could say that all I want is the second word of the string with exactly one space between first & second word. For anything else, I want a null output.

> input_string = "smart" or input_string = "smart ", output_string = nil (null)

This is not possible using gsub alone. gsub will always produce a string. If you want the empty string "" to be converted to nil, you will have to write extra logic: if output_string == "" then output_string = nil end

> input_string = "smart" is erroneously converted to output_string = "smart"

Your pattern contains a trailing space. For it to match, your input must contain smart followed by a space. Just smart does not end with a space.

Make the space optional: smart ? will greedily try to match a space, but will also accept no space. Caveat: This will also convert smartass to ass.

Matching end-of-string or a space as would be possible in RegEx using ( |$) is not possible with patterns since alternation (|) is not supported.

Alternatively, you can use extra logic by just handling "smart" explicitly: if input_string == "smart" then output_string = "" end. This will deal properly with smartass, keeping it intact.

Note that you might want %s instead of a literal space to accept other spacing such as tabs. You'll also probably want a quantifier like * to accept multiple space characters. I'd also add the ^ anchor to the beginning of your pattern to not replace smart anywhere in the string.

All in all, I'd recommend output_string = input_string:gsub("^smart%s*", "")

Regarding your update:

> input_string = "smr" or "smr "
>
> I mean any other word, then also I expect output_string = nil
> You could say that all I want is the second word of the string with exactly one space between first & second word. For anything else, I want a null output.

In that case, you definitely want output_string = input_string:match("^smart (.+)").

Explanation: This expects your string to start with smart and then greedily captures the remaining characters till the end. If you want to support an empty word if the string is just "smart ", use .* instead of .+.

If the string does not start with smart , this won't match and the result will be nil.