Remove dynamic input used in regular expression

lib/datadog/core/backport.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+module Datadog
+  module Core
+    # This module is used to provide features from Ruby 2.5+ to older Rubies
+    module BackportFrom25
+      module_function
+
+      if ::String.method_defined?(:delete_prefix)
+        def string_delete_prefix(string, prefix)
+          string.delete_prefix(prefix)
+        end
+      else
+        def string_delete_prefix(string, prefix)
+          prefix = prefix.to_s
+          if string.start_with?(prefix)
+            string[prefix.length..-1] || raise('rbs-guard: String#[] is non-nil as `prefix` is guaranteed present')
+          else
+            string.dup
+          end
+        end
+      end
+    end
+  end
+end

lib/datadog/tracing/contrib/rack/middlewares.rb

@@ -1,6 +1,7 @@
 require 'date'
 
 require_relative '../../../core/environment/variable_helpers'
+require_relative '../../../core/backport'
 require_relative '../../client_ip'
 require_relative '../../metadata/ext'
 require_relative '../../propagation/http'
@@ -304,7 +305,7 @@ def parse_url(env, original_env)
                        else
                          # normally REQUEST_URI starts at the path, but it
                          # might contain the full URL in some cases (e.g WEBrick)
-                         request_uri.sub(/^#{base_url}/, '')
+                         Datadog::Core::BackportFrom25.string_delete_prefix(request_uri, base_url)
                        end
 
             base_url + fullpath

sig/datadog/core/backport.rbs

@@ -0,0 +1,7 @@
+module Datadog
+  module Core
+    module BackportFrom25
+      def self?.string_delete_prefix: (::String string, ::String prefix) -> ::String
+    end
+  end
+end