diff --git a/lib/rexml/source.rb b/lib/rexml/source.rb
index b0b89b71..2409f76e 100644
--- a/lib/rexml/source.rb
+++ b/lib/rexml/source.rb
@@ -68,8 +68,14 @@ module Private
       SCANNER_RESET_SIZE = 100000
       PRE_DEFINED_TERM_PATTERNS = {}
       pre_defined_terms = ["'", '"', "<"]
-      pre_defined_terms.each do |term|
-        PRE_DEFINED_TERM_PATTERNS[term] = /#{Regexp.escape(term)}/
+      if StringScanner::Version < "3.1.1"
+        pre_defined_terms.each do |term|
+          PRE_DEFINED_TERM_PATTERNS[term] = /#{Regexp.escape(term)}/
+        end
+      else
+        pre_defined_terms.each do |term|
+          PRE_DEFINED_TERM_PATTERNS[term] = term
+        end
       end
     end
     private_constant :Private