Index: org/jruby/ast/DRegexpNode.java
===================================================================
--- org/jruby/ast/DRegexpNode.java	(revision 4911)
+++ org/jruby/ast/DRegexpNode.java	(working copy)
@@ -32,6 +32,7 @@
  ***** END LICENSE BLOCK *****/
 package org.jruby.ast;
 
+import org.jruby.RubyRegexp;
 import org.jruby.ast.types.ILiteralNode;
 import org.jruby.ast.visitor.NodeVisitor;
 import org.jruby.evaluator.Instruction;
@@ -44,17 +45,14 @@
 public class DRegexpNode extends ListNode implements ILiteralNode {
     private final int options;
     private final boolean once;
+    private RubyRegexp onceRegexp;
     
     public DRegexpNode(ISourcePosition position) {
         this(position, 0, false);
     }
 
     public DRegexpNode(ISourcePosition position, DStrNode node, int options, boolean once) {
-        super(position, NodeType.DREGEXPNODE);
-
-        this.options = options;
-        this.once = once;
-        
+        this(position, options, once);
         addAll(node);
     }
 
@@ -88,4 +86,21 @@
     public int getOptions() {
         return options;
     }
+
+    /**
+     * For regular expressions with /o flag
+     * @return
+     */
+    public RubyRegexp getOnceRegexp() {
+        return onceRegexp;
+    }
+    
+    /**
+     * For regular expressions with /o flag, the value in here can be used for subsequent evaluations.
+     * Setting will only succeed if it is a regular expression with /o flag, and the value hasn't been already set.
+     * @param regexp
+     */
+    public void setOnceRegexp(RubyRegexp regexp) {
+        if (once && onceRegexp == null) this.onceRegexp = regexp;
+    }
 }
Index: org/jruby/evaluator/ASTInterpreter.java
===================================================================
--- org/jruby/evaluator/ASTInterpreter.java	(revision 4911)
+++ org/jruby/evaluator/ASTInterpreter.java	(working copy)
@@ -871,7 +871,15 @@
 
     private static IRubyObject dregexpNode(Ruby runtime, ThreadContext context, Node node, IRubyObject self, Block aBlock) {
         DRegexpNode iVisited = (DRegexpNode) node;
-   
+        
+        RubyRegexp regexp;
+        if (iVisited.getOnce()) {
+            regexp = iVisited.getOnceRegexp();
+            if (regexp != null) {
+                return regexp;
+            }
+        }
+
         RubyString string = runtime.newString(new ByteList());
         for (int i = 0; i < iVisited.size(); i++) {
             Node iterNode = iVisited.get(i);
@@ -893,12 +901,18 @@
         }
         
         try {
-            return RubyRegexp.newRegexp(runtime, string.toString(), iVisited.getOptions(), lang);
+            regexp = RubyRegexp.newRegexp(runtime, string.toString(), iVisited.getOptions(), lang);
         } catch(jregex.PatternSyntaxException e) {
         //                    System.err.println(iVisited.getValue().toString());
         //                    e.printStackTrace();
             throw runtime.newRegexpError(e.getMessage());
         }
+        
+        if (iVisited.getOnce()) {
+            iVisited.setOnceRegexp(regexp);
+        }
+
+        return regexp;
     }
     
     private static IRubyObject dStrNode(Ruby runtime, ThreadContext context, Node node, IRubyObject self, Block aBlock) {