diff --git a/src/columns/ColUDA.cpp b/src/columns/ColUDA.cpp
index 7865a19b3..9a4016219 100644
--- a/src/columns/ColUDA.cpp
+++ b/src/columns/ColUDA.cpp
@@ -47,7 +47,7 @@ ColumnUDAString::ColumnUDAString ()
 }
 
 ////////////////////////////////////////////////////////////////////////////////
-bool ColumnUDAString::validate (std::string& value)
+bool ColumnUDAString::validate (const std::string& value) const
 {
   // No restrictions.
   if (_values.size () == 0)
@@ -143,7 +143,7 @@ ColumnUDANumeric::ColumnUDANumeric ()
 }
 
 ////////////////////////////////////////////////////////////////////////////////
-bool ColumnUDANumeric::validate (std::string& value)
+bool ColumnUDANumeric::validate (const std::string& value) const
 {
   // No restrictions.
   if (_values.size () == 0)
@@ -231,7 +231,7 @@ ColumnUDADate::ColumnUDADate ()
 }
 
 ////////////////////////////////////////////////////////////////////////////////
-bool ColumnUDADate::validate (std::string& value)
+bool ColumnUDADate::validate (const std::string& value) const
 {
   // No restrictions.
   if (_values.size () == 0)
@@ -345,7 +345,7 @@ ColumnUDADuration::ColumnUDADuration ()
 }
 
 ////////////////////////////////////////////////////////////////////////////////
-bool ColumnUDADuration::validate (std::string& value)
+bool ColumnUDADuration::validate (const std::string& value) const
 {
   // No restrictions.
   if (_values.size () == 0)
diff --git a/src/columns/ColUDA.h b/src/columns/ColUDA.h
index 362efc881..8d22ee976 100644
--- a/src/columns/ColUDA.h
+++ b/src/columns/ColUDA.h
@@ -37,7 +37,7 @@ class ColumnUDAString : public ColumnTypeString
 {
 public:
   ColumnUDAString ();
-  bool validate (std::string&);
+  bool validate (const std::string&) const;
   void measure (Task&, unsigned int&, unsigned int&);
   void render (std::vector <std::string>&, Task&, int, Color&);
 
@@ -53,7 +53,7 @@ class ColumnUDANumeric : public ColumnTypeNumeric
 {
 public:
   ColumnUDANumeric ();
-  bool validate (std::string&);
+  bool validate (const std::string&) const;
   void measure (Task&, unsigned int&, unsigned int&);
   void render (std::vector <std::string>&, Task&, int, Color&);
 
@@ -66,7 +66,7 @@ class ColumnUDADate : public ColumnTypeDate
 {
 public:
   ColumnUDADate ();
-  bool validate (std::string&);
+  bool validate (const std::string&) const;
   void measure (Task&, unsigned int&, unsigned int&);
   void render (std::vector <std::string>&, Task&, int, Color&);
 
@@ -79,7 +79,7 @@ class ColumnUDADuration : public ColumnTypeDuration
 {
 public:
   ColumnUDADuration ();
-  bool validate (std::string&);
+  bool validate (const std::string&) const;
   void measure (Task&, unsigned int&, unsigned int&);
   void render (std::vector <std::string>&, Task&, int, Color&);
 
diff --git a/src/columns/Column.cpp b/src/columns/Column.cpp
index d2644cb41..c3b1b8e4c 100644
--- a/src/columns/Column.cpp
+++ b/src/columns/Column.cpp
@@ -283,7 +283,7 @@ void Column::setStyle (const std::string& style)
 }
 
 ////////////////////////////////////////////////////////////////////////////////
-bool Column::validate (std::string& input)
+bool Column::validate (const std::string& input) const
 {
   return input.length () ? true : false;
 }
diff --git a/src/columns/Column.h b/src/columns/Column.h
index 9acb3919f..d064a261e 100644
--- a/src/columns/Column.h
+++ b/src/columns/Column.h
@@ -57,7 +57,7 @@ public:
   virtual void setLabel  (const std::string& value) { _label = value;  }
   virtual void setReport (const std::string& value) { _report = value; }
 
-  virtual bool validate (std::string&);
+  virtual bool validate (const std::string&) const;
   virtual void measure (const std::string&, unsigned int&, unsigned int&)           {};
   virtual void measure (Task&, unsigned int&, unsigned int&)                        {};
   virtual void renderHeader (std::vector <std::string>&, int, Color&);