@script Depends for your use case there are few solutions. For some of them Regex is not even required.

Answering your question without directly:

String.replace("1000 cfu/ml", ~r"[a-z /]", "")

Here are some example other solutions:

#1 String.slice/3:

string = "1000 cfu/ml"
String.slice(string, 0, String.length(string) - 7)

#2 String.split/2:

"1000 cfu/ml" |> String.split(" ") |> List.first()

#3 not integer Regex:

"1000 cfu/ml" |> String.split(~r"[^\d]", trim: true) |> List.first()

#4 Integer.parse/1:

Integer.parse("1000 cfu/ml")                      
# {1000, " cfu/ml"}

#5 Regex.scan/2:

~r"\d+" |> Regex.scan("1000 cfu/ml") |> List.first() |> List.first()

#6 Custom pattern-maching:

defmodule Example do
  @codepoints ?0..?9

  def sample(<<>>), do: <<>>
  def sample(<<char::utf8, rest::binary>>) when char in @codepoints, do: <<char::utf8>> <> sample(rest)
  def sample(<<_char::utf8, rest::binary>>), do: sample(rest)
end
# or
defmodule Example do
  @codepoints Enum.to_list(?a..?z) ++ Enum.to_list(?A..?Z) ++ [?\s, ?/]

  def sample(<<>>), do: <<>>
  def sample(<<char::utf8, rest::binary>>) when char not in @codepoints, do: <<char::utf8>> <> sample(rest)
  def sample(<<_char::utf8, rest::binary>>), do: sample(rest)
end

and many more …