Keeping underscore between digits on Macro.camelize/1 (#10843)

Closes #10839

Author: kelvinst

lib/elixir/lib/macro.ex

@@ -1737,6 +1737,9 @@ defmodule Macro do
   defp do_camelize(<<?_, h, t::binary>>) when h >= ?a and h <= ?z,
     do: <<to_upper_char(h)>> <> do_camelize(t)
 
+  defp do_camelize(<<p, ?_, h, t::binary>>) when p >= ?0 and p <= ?9 and h >= ?0 and h <= ?9,
+    do: <<p, ?_, h>> <> do_camelize(t)
+
   defp do_camelize(<<?_, h, t::binary>>) when h >= ?0 and h <= ?9, do: <<h>> <> do_camelize(t)
   defp do_camelize(<<?_>>), do: <<>>
   defp do_camelize(<<?/, t::binary>>), do: <<?.>> <> camelize(t)

lib/elixir/test/elixir/macro_test.exs

@@ -1036,6 +1036,7 @@ defmodule MacroTest do
     assert Macro.camelize("foo__bar") == "FooBar"
     assert Macro.camelize("foo/bar") == "Foo.Bar"
     assert Macro.camelize("Foo.Bar") == "Foo.Bar"
+    assert Macro.camelize("foo1_0") == "Foo1_0"
     assert Macro.camelize("FOO_BAR") == "FOO_BAR"
     assert Macro.camelize("FOO.BAR") == "FOO.BAR"
     assert Macro.camelize("") == ""