fix migration

db/migrate/20180912185624_add_prefixes_to_tags.rb

@@ -1,6 +1,6 @@
 class AddPrefixesToTags < ActiveRecord::Migration[5.2]
   def change
     execute "set statement_timeout = 0"
-    execute "CREATE INDEX index_tags_on_name_prefix ON tags USING gin (REGEXP_REPLACE(name, '([a-z0-9])[a-z0-9'']*($|[^a-z0-9'']+)', '\1', 'g') gin_trgm_ops) WHERE post_count > 0"
+    execute "CREATE INDEX index_tags_on_name_prefix ON tags USING gin (REGEXP_REPLACE(name, '([a-z0-9])[a-z0-9'']*($|[^a-z0-9'']+)', '\\1', 'g') gin_trgm_ops) WHERE post_count > 0"
   end
 end