Improve performance by avoiding regex construction (#20215)

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

4 files changed, 13 insertions, 8 deletions

diff --git a/app/models/account.rb b/app/models/account.rb
index be1968fa6..cc3a8f3df 100644
--- a/app/models/account.rb
+++ b/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' }
diff --git a/app/models/custom_emoji.rb b/app/models/custom_emoji.rb
index 077ce559a..7b19cd2ac 100644
--- a/app/models/custom_emoji.rb
+++ b/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) }
diff --git a/app/models/featured_tag.rb b/app/models/featured_tag.rb
index debae2212..70f949b6a 100644
--- a/app/models/featured_tag.rb
+++ b/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
diff --git a/app/models/tag.rb b/app/models/tag.rb
index 8929baf66..b66f85423 100644
--- a/app/models/tag.rb
+++ b/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?