Optimize some regex matching (#15528)

* Use Regex#match?

* Replace =~ too

* Avoid to call match? from Nil

* Keep value of Regexp.last_match

app/lib/feed_manager.rb

@@ -396,8 +396,8 @@ class FeedManager
 
     active_filters.map! do |filter|
       if filter.whole_word
-        sb = filter.phrase =~ /\A[[:word:]]/ ? '\b' : ''
-        eb = filter.phrase =~ /[[:word:]]\z/ ? '\b' : ''
+        sb = /\A[[:word:]]/.match?(filter.phrase) ? '\b' : ''
+        eb = /[[:word:]]\z/.match?(filter.phrase) ? '\b' : ''
 
         /(?mix:#{sb}#{Regexp.escape(filter.phrase)}#{eb})/
       else
@@ -417,7 +417,7 @@ class FeedManager
       status.media_attachments.map(&:description).join("\n\n"),
     ].compact.join("\n\n")
 
-    !combined_regex.match(combined_text).nil?
+    combined_regex.match?(combined_text)
   end
 
   # Adds a status to an account's feed, returning true if a status was