diff --git a/alib/src/regexp/Alternation.h b/alib/src/regexp/Alternation.h
index e01aac375cc983a62132e240ede84e8b4d06837f..d5f5c94622140975754787c63b6e832004845260 100644
--- a/alib/src/regexp/Alternation.h
+++ b/alib/src/regexp/Alternation.h
@@ -54,12 +54,12 @@ public:
 	/**
 	 * @copydoc RegExpElement::containsEmptyString() const
 	 */
-	bool containsEmptyString() const;
+	virtual bool containsEmptyString() const;
 
 	/**
 	 * @copydoc RegExpElement::isEmpty() const
 	 */
-	bool isEmpty() const;
+	virtual bool isEmpty() const;
 };
 
 } /* namespace regexp */
diff --git a/alib/src/regexp/Concatenation.h b/alib/src/regexp/Concatenation.h
index 55b465ed9aa4837af53c33238ff1dd5b8d9b3625..955c6b765b7667fb1fd624b1236e866367450a79 100644
--- a/alib/src/regexp/Concatenation.h
+++ b/alib/src/regexp/Concatenation.h
@@ -53,12 +53,12 @@ public:
 	/**
 	 * @copydoc RegExpElement::containsEmptyString() const
 	 */
-	bool containsEmptyString() const;
+	virtual bool containsEmptyString() const;
 
 	/**
 	 * @copydoc RegExpElement::isEmpty() const
 	 */
-	bool isEmpty() const;
+	virtual bool isEmpty() const;
 };
 
 } /* namespace regexp */
diff --git a/alib/src/regexp/Iteration.h b/alib/src/regexp/Iteration.h
index f7b755113cae3a193680cf9b318bedf561e0be5a..f5e1f2c5c60e8c89481dfb0ce06be90f8feb4548 100644
--- a/alib/src/regexp/Iteration.h
+++ b/alib/src/regexp/Iteration.h
@@ -61,12 +61,12 @@ public:
 	/**
 	 * @copydoc RegExpElement::containsEmptyString() const
 	 */
-	bool containsEmptyString() const;
+	virtual bool containsEmptyString() const;
 
 	/**
 	 * @copydoc RegExpElement::isEmpty() const
 	 */
-	bool isEmpty() const;
+	virtual bool isEmpty() const;
 };
 
 } /* namespace regexp */
diff --git a/alib/src/regexp/RegExpEmpty.h b/alib/src/regexp/RegExpEmpty.h
index 3578afa59f530be6fe5ee65ecdd18f526b7a4087..f56089d6c88360e9affffadd74136c57181d23fd 100644
--- a/alib/src/regexp/RegExpEmpty.h
+++ b/alib/src/regexp/RegExpEmpty.h
@@ -40,12 +40,12 @@ public:
 	/**
 	 * @copydoc RegExpElement::containsEmptyString() const
 	 */
-	bool containsEmptyString() const;
+	virtual bool containsEmptyString() const;
 
 	/**
 	 * @copydoc RegExpElement::isEmpty() const
 	 */
-	bool isEmpty() const;
+	virtual bool isEmpty() const;
 };
 
 } /* namespace regexp */
diff --git a/alib/src/regexp/RegExpEpsilon.h b/alib/src/regexp/RegExpEpsilon.h
index 28b8a6c0a432ac69fe0773f0c86200d814ac6109..a3dc873544d4497f5147fbccdcadf15db61f8f89 100644
--- a/alib/src/regexp/RegExpEpsilon.h
+++ b/alib/src/regexp/RegExpEpsilon.h
@@ -41,12 +41,12 @@ public:
 	/**
 	 * @copydoc RegExpElement::containsEmptyString() const
 	 */
-	bool containsEmptyString() const;
+	virtual bool containsEmptyString() const;
 
 	/**
 	 * @copydoc RegExpElement::isEmpty() const
 	 */
-	bool isEmpty() const;
+	virtual bool isEmpty() const;
 };
 
 } /* namespace regexp */
diff --git a/alib/src/regexp/RegExpSymbol.h b/alib/src/regexp/RegExpSymbol.h
index 54e1b618aaa423dad5f4a93ae69904ca5f0ab17a..ce0a6b60552c688c488819d4fb11a388efbd2d71 100644
--- a/alib/src/regexp/RegExpSymbol.h
+++ b/alib/src/regexp/RegExpSymbol.h
@@ -45,14 +45,14 @@ public:
 	/**
 	 * @copydoc RegExpElement::containsEmptyString() const
 	 */
-	bool containsEmptyString() const;
+	virtual bool containsEmptyString() const;
 
 	const string& getSymbol() const;
 
 	/**
 	 * @copydoc RegExpElement::isEmpty() const
 	 */
-	bool isEmpty() const;
+	virtual bool isEmpty() const;
 };
 
 } /* namespace regexp */