diff --git a/app/models/projects/identifier.rb b/app/models/projects/identifier.rb
index 0649ce411cfd..b9835e3aff53 100644
--- a/app/models/projects/identifier.rb
+++ b/app/models/projects/identifier.rb
@@ -36,6 +36,10 @@ module Projects::Identifier
   # Classic identifier format: lowercase letters, digits, hyphens, underscores — but not all-numeric.
   CLASSIC_IDENTIFIER_FORMAT = /\A(?!\d+\z)[a-z0-9\-_]+\z/
 
+  # Unanchored shape of a semantic project identifier ("PROJ", "MY_PROJECT_1").
+  # Composed into `WorkPackage::SemanticIdentifier::SEMANTIC_ID_PATTERN`.
+  SEMANTIC_FORMAT = /[A-Z][A-Z0-9_]*/
+
   RESERVED_IDENTIFIERS = %w[new menu queries filters identifier_update_dialog identifier_suggestion].freeze
 
   included do
diff --git a/app/models/work_package/semantic_identifier.rb b/app/models/work_package/semantic_identifier.rb
index a1608d74860f..cfc0e528e73e 100644
--- a/app/models/work_package/semantic_identifier.rb
+++ b/app/models/work_package/semantic_identifier.rb
@@ -31,10 +31,15 @@
 module WorkPackage::SemanticIdentifier
   extend ActiveSupport::Concern
 
+  # Semantic-identifier shape ("PROJ-42"). Use this when the numeric and
+  # semantic branches need different boundary rules; use `ID_ROUTE_CONSTRAINT`
+  # when both branches share one regex.
+  SEMANTIC_ID_PATTERN = /#{Projects::Identifier::SEMANTIC_FORMAT.source}-\d+/
+
   # Matches either a numeric ID ("12345") or a semantic identifier ("PROJ-42").
   # Used in Rails route constraints so both forms are accepted in URLs.
   # The frontend equivalent lives in WP_ID_URL_PATTERN (work-package-id-pattern.ts).
-  ID_ROUTE_CONSTRAINT = /\d+|[A-Z][A-Z0-9_]*-\d+/
+  ID_ROUTE_CONSTRAINT = /\d+|#{SEMANTIC_ID_PATTERN.source}/
 
   # Raised when a finder is invoked in a way that cannot resolve a semantic
   # identifier — e.g. find_by(id: "PROJ-42") which reduces to a raw SQL
diff --git a/app/validators/projects/identifier_validator.rb b/app/validators/projects/identifier_validator.rb
index c1703d68e442..4960a37b9b12 100644
--- a/app/validators/projects/identifier_validator.rb
+++ b/app/validators/projects/identifier_validator.rb
@@ -30,6 +30,9 @@
 
 module Projects
   class IdentifierValidator < ActiveModel::EachValidator
+    # Two anchored patterns, one per rule, so the start-character and body
+    # checks produce distinct error messages. The full unanchored shape
+    # lives at `Projects::Identifier::SEMANTIC_FORMAT`.
     SEMANTIC_START_FORMAT = /\A[A-Z]/
     SEMANTIC_BODY_FORMAT  = /\A[A-Z0-9_]*\z/