fix(filters): treat phrases as literal text in the regex

Phrases went into the Postgres and Elixir regexes as they were, so a
phrase like "(" broke the timeline and notification queries. The :re
form also put the word boundaries only around the whole alternation.

Escape each phrase and, as Mastodon does, add a word boundary only where
the phrase starts or ends with a word character.

Reported in Akkoma (d6209837b).

test/pleroma/filter_test.exs

@@ -190,4 +190,55 @@ defmodule Pleroma.FilterTest do
 
     {:error, _} = Filter.update(filter, changes)
   end
+
+  describe "compose_regex/2" do
+    defp filters(phrases), do: Enum.map(phrases, &%Filter{phrase: &1})
+
+    defp pg_match?(regex, text) do
+      %{rows: [[match]]} = Repo.query!("SELECT $1::text ~* $2", [text, regex])
+      match
+    end
+
+    test "treats phrases as literal text in Postgres" do
+      regex = Filter.compose_regex(filters(["a.b", "(c"]))
+
+      assert pg_match?(regex, "see a.b here")
+      refute pg_match?(regex, "see axb here")
+      assert pg_match?(regex, "see (c here")
+    end
+
+    test "treats phrases as literal text in Elixir" do
+      regex = Filter.compose_regex(filters(["a.b", "(c"]), :re)
+
+      assert Regex.match?(regex, "see a.b here")
+      refute Regex.match?(regex, "see axb here")
+      assert Regex.match?(regex, "see (c here")
+    end
+
+    test "keeps word boundaries for every phrase in Postgres" do
+      regex = Filter.compose_regex(filters(["cat", "dog"]))
+
+      assert pg_match?(regex, "a dog barks")
+      refute pg_match?(regex, "a catalog")
+      refute pg_match?(regex, "a hotdog")
+    end
+
+    test "only adds word boundaries next to word characters" do
+      pg = Filter.compose_regex(filters(["#tag"]))
+      re = Filter.compose_regex(filters(["#tag"]), :re)
+
+      for {text, expected} <- [{"a #tag here", true}, {"a#tag", true}, {"some #tags", false}] do
+        assert pg_match?(pg, text) == expected
+        assert Regex.match?(re, text) == expected
+      end
+    end
+
+    test "keeps word boundaries for every phrase in Elixir" do
+      regex = Filter.compose_regex(filters(["cat", "dog"]), :re)
+
+      assert Regex.match?(regex, "a dog barks")
+      refute Regex.match?(regex, "a catalog")
+      refute Regex.match?(regex, "a hotdog")
+    end
+  end
 end