# Potential Elixir/Erlang bug

**URL:** <https://forum.elixirforum.com/t/potential-elixir-erlang-bug/40456>\
**Category:** Questions\
**Tags:** regex, troubleshooting\
**Created:** [June 15, 2021, 2:44pm UTC](https://forum.elixirforum.com/t/potential-elixir-erlang-bug/40456 "2021-06-15T14:44:17Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![tomekowal](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/tomekowal/32/2847_2.png) [@tomekowal](https://forum.elixirforum.com/u/tomekowal)\
**Post date:** [June 15, 2021, 2:44pm UTC](https://forum.elixirforum.com/t/potential-elixir-erlang-bug/40456/1 "2021-06-15T14:44:17Z")

</div>

I have a bug that started randomly failing our tests on Heroku.  
In a very particular scenario once every couple hundred of runs `Regex.split(~r/\s+/, "10 EUR")` returns a one-element list `["10.00 EUR"]` instead of two-element list `["10.00", "EUR"]`. If I call it directly, it always works. However, in this very particular setup presented in this repo, it reliably fails every now and then.

I can see the behaviour both locally and on Heroku so I am starting to believe I’ve detected a bug in Elixir.

If anyone is interested, here is a repo with a minimal test setup.  
Let me know if you can reproduce it with instructions from the repo and if you have any idea about how to fix it.

[https://github.com/socialpaymentsbv/elixir\_bug](https://github.com/socialpaymentsbv/elixir_bug)

---

<div class="post-metadata">

**Author:** ![patrickdm](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/patrickdm/32/7579_2.png) [@patrickdm](https://forum.elixirforum.com/u/patrickdm)\
**Post date:** [June 15, 2021, 3:11pm UTC](https://forum.elixirforum.com/t/potential-elixir-erlang-bug/40456/2 "2021-06-15T15:11:35Z")

</div>

I could reproduce it using your test repo with erlang 23.3 and elixir 1.10.4-otp-23,

```elixir
[..]
STR: "10.00 EUR"
LST: ["10.00", "EUR"]
VALUE: "10.00"
STR: "10.00 EUR"
LST: ["10.00 EUR"]
VALUE: ""

  1) test this test fails every couple of runs (BugTest)
     test/bug_test.exs:5
     ** (MatchError) no match of right hand side value: :error
     code: for x <- 1..1000 do
     stacktrace:
       test/bug_test.exs:7: anonymous fn/2 in BugTest."test this test fails every couple of runs"/1
       (elixir 1.10.4) lib/enum.ex:3371: Enum.reduce_range_inc/4
       test/bug_test.exs:6: (test)
.
Finished in 0.05 seconds
1 doctest, 1 test, 1 failure

```

I have no idea of the cause, but out of curiosity I added a

`IO.puts(x)` to the test, to check when the error occurred

```elixir
  test "this test fails every couple of runs" do
    for x <- 1..1000 do
      IO.puts(x)
      {:ok, m} = Money.cast(%{amount: "10.00", currency: "EUR"})
    end
  end

```

and the error disappeared. 😳 (are we really living in a simulation then? 😏)

---

<div class="post-metadata">

**Author:** ![kip](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/kip/32/1440_2.png) [@kip](https://forum.elixirforum.com/u/kip)\
**Post date:** [June 15, 2021, 3:14pm UTC](https://forum.elixirforum.com/t/potential-elixir-erlang-bug/40456/3 "2021-06-15T15:14:26Z")

</div>

I couldn’t reproduce either with your repo or a simple `benchee` script:

```elixir
Benchee.run(
  %{
    "Regex.split/2" =>
      fn ->
        [_a, _b] = Regex.split(~r/\s+/, "10.00 EUR")
      end,
    },
  time: 10,
  memory_time: 2
)

```

Which ran for over a million iterations

I’m on:

```elixir
Erlang/OTP 24 [erts-12.0.2] [source] [64-bit] [smp:16:16] [ds:16:16:10] [async-threads:1] [jit]

Interactive Elixir (1.12.1) - press Ctrl+C to exit (type h() ENTER for help)

```

---

<div class="post-metadata">

**Author:** ![tomekowal](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/tomekowal/32/2847_2.png) [@tomekowal](https://forum.elixirforum.com/u/tomekowal)\
**Post date:** [June 15, 2021, 3:50pm UTC](https://forum.elixirforum.com/t/potential-elixir-erlang-bug/40456/4 "2021-06-15T15:50:24Z")

</div>

Yes, I questioned my sanity too 😃

---

<div class="post-metadata">

**Author:** ![tomekowal](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/tomekowal/32/2847_2.png) [@tomekowal](https://forum.elixirforum.com/u/tomekowal)\
**Post date:** [June 15, 2021, 3:54pm UTC](https://forum.elixirforum.com/t/potential-elixir-erlang-bug/40456/5 "2021-06-15T15:54:11Z")

</div>

Just tried erlang 24.0.2 and elixir 1.12.1 and the bug disappeared. I’ll try to pinpoint which upgrade fixes the issue.

---

<div class="post-metadata">

**Author:** ![outlog](https://forum.elixirforum.com/letter_avatar_proxy/v4/letter/o/c0e974/32.png) [@outlog](https://forum.elixirforum.com/u/outlog)\
**Post date:** [June 15, 2021, 4:02pm UTC](https://forum.elixirforum.com/t/potential-elixir-erlang-bug/40456/6 "2021-06-15T16:02:06Z")

</div>

I can reproduce on some combinations of elixir/erlang..

never on otp 24  
never on elixir 1.11 and above (on otp 23 or 24)

seems specific to the otp-23 builds eg elixir 1.10.4-otp-23 will fail - but 1.10.4-otp-22 will not (both against otp 23.3.4.3)

---

<div class="post-metadata">

**Author:** ![kip](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/kip/32/1440_2.png) [@kip](https://forum.elixirforum.com/u/kip)\
**Post date:** [June 15, 2021, 4:09pm UTC](https://forum.elixirforum.com/t/potential-elixir-erlang-bug/40456/7 "2021-06-15T16:09:48Z")

</div>

Perhaps can be traced by:

#### OTP 24

```elixir
iex> :re.version
"8.44 2020-02-12"

```

#### OTP 23

```elixir
iex> :re.version
"8.43 2019-02-23"

```

#### OTP 22

```erlang
1> re:version().
<<"8.43 2019-02-23">>

```

#### OTP 21

```erlang
1> re:version().
<<"8.42 2018-03-20">>

```