Fix Macro.underscore for digit preceded by capitals, closes #10713 (#10715)

Author: vhf

lib/elixir/lib/macro.ex

@@ -1671,7 +1671,8 @@ defmodule Macro do
   end
 
   defp do_underscore(<<h, t, rest::binary>>, _)
-       when h >= ?A and h <= ?Z and not (t >= ?A and t <= ?Z) and t != ?. and t != ?_ do
+       when h >= ?A and h <= ?Z and not (t >= ?A and t <= ?Z) and not (t >= ?0 and t <= ?9) and
+              t != ?. and t != ?_ do
     <<?_, to_lower_char(h), t>> <> do_underscore(rest, t)
   end
 

lib/elixir/test/elixir/macro_test.exs

@@ -1007,8 +1007,10 @@ defmodule MacroTest do
     assert Macro.underscore("FOO_BAR") == "foo_bar"
     assert Macro.underscore("FoBaZa") == "fo_ba_za"
     assert Macro.underscore("Foo10") == "foo10"
+    assert Macro.underscore("FOO10") == "foo10"
     assert Macro.underscore("10Foo") == "10_foo"
     assert Macro.underscore("FooBar10") == "foo_bar10"
+    assert Macro.underscore("FooBAR10") == "foo_bar10"
     assert Macro.underscore("Foo10Bar") == "foo10_bar"
     assert Macro.underscore("Foo.Bar") == "foo/bar"
     assert Macro.underscore(Foo.Bar) == "foo/bar"