Split a string using string.gmatch() in Lua

Question

There are some discussions here, and utility functions, for splitting strings, but I need an ad-hoc one-liner for a very simple task.

I have the following string:

local s = "one;two;;four"

And I want to split it on ";". I want, eventually, go get { "one", "two", "", "four" } in return.

So I tried to do:

local s = "one;two;;four"

local words = {}
for w in s:gmatch("([^;]*)") do table.insert(words, w) end

But the result (the words table) is { "one", "", "two", "", "", "four", "" }. That's certainly not what I want.

Now, as I remarked, there are some discussions here on splitting strings, but they have "lengthy" functions in them and I need something succinct. I need this code for a program where I show the merit of Lua, and if I add a lengthy function to do something so trivial it would go against me.

[^;]* is perfectly happy matching zero semicolons. So lua matches zero semicolons each time it gets to a delimiter. You can use "[^;]+" instead for a slightly better result but there are reasons the lua-users.org/wiki/SplitJoin page of the lua-users wiki runs as long as it does when talking about splitting strings. — Etan Reisner
– Etan Reisner, Commented Nov 11, 2013 at 14:18

Yu Hao · Accepted Answer · 2013-11-11 13:57:36Z

24

local s = "one;two;;four"
local words = {}
for w in (s .. ";"):gmatch("([^;]*);") do 
    table.insert(words, w) 
end

By adding one extra ; at the end of the string, the string now becomes "one;two;;four;", everything you want to capture can use the pattern "([^;]*);" to match: anything not ; followed by a ;(greedy).

Test:

for n, w in ipairs(words) do
    print(n .. ": " .. w)
end

Output:

1: one
2: two
3:
4: four

answered Nov 11, 2013 at 13:57

Yu Hao

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6 Comments

Niccolo M. Over a year ago

Wow, thanks. Your solution works perfectly! (I won't close this question yet: if somebody could explain to me why my original code returns spurious empty strings I'd be grateful.)

Yu Hao Over a year ago

@NiccoloM. Remember that * matches zero or more, the empty string where I marked $ in the string one$;two$;$;four$ is also a match.

Niccolo M. Over a year ago

But what about one$;$two$;$;$fo$ur$? Why is the zero match only before ; ? Why isn't it also after the ;, and between every two letters?

Yu Hao Over a year ago

@NiccoloM. Because * is greedy, it will try to match as long as possible, the non-greedy version to match zero or more is -.

wilson0x4d Over a year ago

It's worth noting that LUA Patterns are not actually Regular Expressions, you will notice many differences between 'standard' regexp implementations and how LUA patterns operate.

|

Heywood Jason · Accepted Answer · 2020-12-31 09:04:16Z

0

Just changing * to + works.

local s = "one;two;;four"
local words = {}
for w in s:gmatch("([^;]+)") do 
    table.insert(words, w) 
    print(w)
end

The magic character * represents 0 or more occurrene, so when it meet ',', lua regarded it as a empty string that [^;] does not exist.

Sorry for my carelessness, the words[3] should be a empty string, but when I run the original code in lua5.4 interpreter, everything works.

code here

running result here (I have to put links because of lack of reputation)

edited Dec 31, 2020 at 9:04

answered Nov 26, 2020 at 13:53

Heywood Jason

11 bronze badge

2 Comments

Nifim Over a year ago

This does not give the OP's desired output, they want an empty string at index 3. { "one", "two", "", "four" }

Heywood Jason Over a year ago

@Nifim sry,I dont read the question carefully.But when I use lua5.4 interpreter, the original code suddenly works!?

han xi · Accepted Answer · 2014-11-24 06:30:31Z

-2

function split(str,sep)
    local array = {}
    local reg = string.format("([^%s]+)",sep)
    for mem in string.gmatch(str,reg) do
        table.insert(array, mem)
    end
    return array
end
local s = "one;two;;four"
local array = split(s,";")

for n, w in ipairs(array) do
    print(n .. ": " .. w)
end

result:

1:one

2:two

3:four

edited Nov 24, 2014 at 6:30

answered Nov 21, 2014 at 2:06

han xi

71 bronze badge

3 Comments

AbcAeffchen Over a year ago

Your answer should contain an explanation of your code and a description how it solves the problem.

Yu Hao Over a year ago

This doesn't work as OP expected, because the empty string between ;; isn't captured.

han xi Over a year ago

I see. Now I understand what you mean.

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