Improve performance by avoiding regex construction (#20215)

```ruby
10.times { p /#{FOO}/.object_id }
10.times { p FOO_RE.object_id }
```

app/controllers/api/v1/tags_controller.rb

@@ -24,7 +24,7 @@ class Api::V1::TagsController < Api::BaseController
   private
 
   def set_or_create_tag
-    return not_found unless /\A(#{Tag::HASHTAG_NAME_RE})\z/.match?(params[:id])
+    return not_found unless Tag::HASHTAG_NAME_RE.match?(params[:id])
     @tag = Tag.find_normalized(params[:id]) || Tag.new(name: Tag.normalize(params[:id]), display_name: params[:id])
   end
 end

app/lib/hashtag_normalizer.rb

@@ -8,7 +8,7 @@ class HashtagNormalizer
   private
 
   def remove_invalid_characters(str)
-    str.gsub(/[^[:alnum:]#{Tag::HASHTAG_SEPARATORS}]/, '')
+    str.gsub(Tag::HASHTAG_INVALID_CHARS_RE, '')
   end
 
   def ascii_folding(str)

app/models/account.rb

@@ -64,6 +64,7 @@ class Account < ApplicationRecord
   USERNAME_RE   = /[a-z0-9_]+([a-z0-9_\.-]+[a-z0-9_]+)?/i
   MENTION_RE    = /(?<=^|[^\/[:word:]])@((#{USERNAME_RE})(?:@[[:word:]\.\-]+[[:word:]]+)?)/i
   URL_PREFIX_RE = /\Ahttp(s?):\/\/[^\/]+/
+  USERNAME_ONLY_RE = /\A#{USERNAME_RE}\z/i
 
   include Attachmentable
   include AccountAssociations
@@ -84,7 +85,7 @@ class Account < ApplicationRecord
   validates_with UniqueUsernameValidator, if: -> { will_save_change_to_username? }
 
   # Remote user validations
-  validates :username, format: { with: /\A#{USERNAME_RE}\z/i }, if: -> { !local? && will_save_change_to_username? }
+  validates :username, format: { with: USERNAME_ONLY_RE }, if: -> { !local? && will_save_change_to_username? }
 
   # Local user validations
   validates :username, format: { with: /\A[a-z0-9_]+\z/i }, length: { maximum: 30 }, if: -> { local? && will_save_change_to_username? && actor_type != 'Application' }

app/models/custom_emoji.rb

@@ -30,6 +30,7 @@ class CustomEmoji < ApplicationRecord
   SCAN_RE = /(?<=[^[:alnum:]:]|\n|^)
     :(#{SHORTCODE_RE_FRAGMENT}):
     (?=[^[:alnum:]:]|$)/x
+  SHORTCODE_ONLY_RE = /\A#{SHORTCODE_RE_FRAGMENT}\z/
 
   IMAGE_MIME_TYPES = %w(image/png image/gif image/webp).freeze
 
@@ -41,7 +42,7 @@ class CustomEmoji < ApplicationRecord
   before_validation :downcase_domain
 
   validates_attachment :image, content_type: { content_type: IMAGE_MIME_TYPES }, presence: true, size: { less_than: LIMIT }
-  validates :shortcode, uniqueness: { scope: :domain }, format: { with: /\A#{SHORTCODE_RE_FRAGMENT}\z/ }, length: { minimum: 2 }
+  validates :shortcode, uniqueness: { scope: :domain }, format: { with: SHORTCODE_ONLY_RE }, length: { minimum: 2 }
 
   scope :local, -> { where(domain: nil) }
   scope :remote, -> { where.not(domain: nil) }

app/models/featured_tag.rb

@@ -17,7 +17,7 @@ class FeaturedTag < ApplicationRecord
   belongs_to :account, inverse_of: :featured_tags
   belongs_to :tag, inverse_of: :featured_tags, optional: true # Set after validation
 
-  validates :name, presence: true, format: { with: /\A(#{Tag::HASHTAG_NAME_RE})\z/i }, on: :create
+  validates :name, presence: true, format: { with: Tag::HASHTAG_NAME_RE }, on: :create
 
   validate :validate_tag_uniqueness, on: :create
   validate :validate_featured_tags_limit, on: :create

app/models/tag.rb

@@ -27,11 +27,14 @@ class Tag < ApplicationRecord
   has_many :followers, through: :passive_relationships, source: :account
 
   HASHTAG_SEPARATORS = "_\u00B7\u200c"
-  HASHTAG_NAME_RE    = "([[:word:]_][[:word:]#{HASHTAG_SEPARATORS}]*[[:alpha:]#{HASHTAG_SEPARATORS}][[:word:]#{HASHTAG_SEPARATORS}]*[[:word:]_])|([[:word:]_]*[[:alpha:]][[:word:]_]*)"
-  HASHTAG_RE         = /(?:^|[^\/\)\w])#(#{HASHTAG_NAME_RE})/i
+  HASHTAG_NAME_PAT = "([[:word:]_][[:word:]#{HASHTAG_SEPARATORS}]*[[:alpha:]#{HASHTAG_SEPARATORS}][[:word:]#{HASHTAG_SEPARATORS}]*[[:word:]_])|([[:word:]_]*[[:alpha:]][[:word:]_]*)"
 
-  validates :name, presence: true, format: { with: /\A(#{HASHTAG_NAME_RE})\z/i }
-  validates :display_name, format: { with: /\A(#{HASHTAG_NAME_RE})\z/i }
+  HASHTAG_RE = /(?:^|[^\/\)\w])#(#{HASHTAG_NAME_PAT})/i
+  HASHTAG_NAME_RE = /\A(#{HASHTAG_NAME_PAT})\z/i
+  HASHTAG_INVALID_CHARS_RE = /[^[:alnum:]#{HASHTAG_SEPARATORS}]/
+
+  validates :name, presence: true, format: { with: HASHTAG_NAME_RE }
+  validates :display_name, format: { with: HASHTAG_NAME_RE }
   validate :validate_name_change, if: -> { !new_record? && name_changed? }
   validate :validate_display_name_change, if: -> { !new_record? && display_name_changed? }
 
@@ -102,7 +105,7 @@ class Tag < ApplicationRecord
       names = Array(name_or_names).map { |str| [normalize(str), str] }.uniq(&:first)
 
       names.map do |(normalized_name, display_name)|
-        tag = matching_name(normalized_name).first || create(name: normalized_name, display_name: display_name.gsub(/[^[:alnum:]#{HASHTAG_SEPARATORS}]/, ''))
+        tag = matching_name(normalized_name).first || create(name: normalized_name, display_name: display_name.gsub(HASHTAG_INVALID_CHARS_RE, ''))
 
         yield tag if block_given?
 

app/services/account_search_service.rb

@@ -3,6 +3,8 @@
 class AccountSearchService < BaseService
   attr_reader :query, :limit, :offset, :options, :account
 
+  MENTION_ONLY_RE = /\A#{Account::MENTION_RE}\z/i
+
   # Min. number of characters to look for non-exact matches
   MIN_QUERY_LENGTH = 5
 
@@ -180,7 +182,7 @@ class AccountSearchService < BaseService
   end
 
   def username_complete?
-    query.include?('@') && "@#{query}".match?(/\A#{Account::MENTION_RE}\Z/)
+    query.include?('@') && "@#{query}".match?(MENTION_ONLY_RE)
   end
 
   def likely_acct?

app/services/resolve_url_service.rb

@@ -4,6 +4,8 @@ class ResolveURLService < BaseService
   include JsonLdHelper
   include Authorization
 
+  USERNAME_STATUS_RE = %r{/@(?<username>#{Account::USERNAME_RE})/(?<status_id>[0-9]+)\Z}
+
   def call(url, on_behalf_of: nil)
     @url          = url
     @on_behalf_of = on_behalf_of
@@ -43,7 +45,7 @@ class ResolveURLService < BaseService
 
     # We don't have an index on `url`, so try guessing the `uri` from `url`
     parsed_url = Addressable::URI.parse(@url)
-    parsed_url.path.match(%r{/@(?<username>#{Account::USERNAME_RE})/(?<status_id>[0-9]+)\Z}) do |matched|
+    parsed_url.path.match(USERNAME_STATUS_RE) do |matched|
       parsed_url.path = "/users/#{matched[:username]}/statuses/#{matched[:status_id]}"
       scope = scope.or(Status.where(uri: parsed_url.to_s, url: @url))
     end