diff --git a/include/CLI/Option.hpp b/include/CLI/Option.hpp
index b44d06d7..81004b3a 100644
--- a/include/CLI/Option.hpp
+++ b/include/CLI/Option.hpp
@@ -148,7 +148,7 @@ class Option : public OptionBase<Option> {
     bool changeable_{false};
 
     /// A list of validators to run on each value parsed
-    std::vector<std::function<bool(std::string)>> validators_;
+    std::vector<std::function<bool(std::string &)>> validators_;
 
     /// A list of options that are required with this option
     std::set<Option *> requires_;
@@ -220,7 +220,7 @@ class Option : public OptionBase<Option> {
     }
 
     /// Adds a validator
-    Option *check(std::function<bool(std::string)> validator) {
+    Option *check(std::function<bool(std::string &)> validator) {
 
         validators_.push_back(validator);
         return this;
@@ -417,7 +417,16 @@ class Option : public OptionBase<Option> {
     ///@{
 
     /// Process the callback
-    void run_callback() const {
+    void run_callback() {
+
+        // Run the validators (can change the string)
+        if(!validators_.empty()) {
+            for(std::string &result : results_)
+                for(const std::function<bool(std::string &)> &vali : validators_)
+                    if(!vali(result))
+                        throw ValidationError("Failed validation: " + get_name() + "=" + result);
+        }
+
         bool local_result;
         // If take_last, only operate on the final item
         if(last_) {
@@ -429,13 +438,6 @@ class Option : public OptionBase<Option> {
 
         if(local_result)
             throw ConversionError("Could not convert: " + get_name() + "=" + detail::join(results_));
-
-        if(!validators_.empty()) {
-            for(const std::string &result : results_)
-                for(const std::function<bool(std::string)> &vali : validators_)
-                    if(!vali(result))
-                        throw ValidationError("Failed validation: " + get_name() + "=" + result);
-        }
     }
 
     /// If options share any of the same names, they are equal (not counting positional)
diff --git a/include/CLI/Validators.hpp b/include/CLI/Validators.hpp
index 975d5c71..f8664087 100644
--- a/include/CLI/Validators.hpp
+++ b/include/CLI/Validators.hpp
@@ -23,7 +23,7 @@ namespace CLI {
 /// @{
 
 /// Check for an existing file
-inline bool ExistingFile(std::string filename) {
+inline bool ExistingFile(const std::string &filename) {
     struct stat buffer;
     bool exist = stat(filename.c_str(), &buffer) == 0;
     bool is_dir = (buffer.st_mode & S_IFDIR) != 0;
@@ -39,7 +39,7 @@ inline bool ExistingFile(std::string filename) {
 }
 
 /// Check for an existing directory
-inline bool ExistingDirectory(std::string filename) {
+inline bool ExistingDirectory(const std::string &filename) {
     struct stat buffer;
     bool exist = stat(filename.c_str(), &buffer) == 0;
     bool is_dir = (buffer.st_mode & S_IFDIR) != 0;
@@ -55,7 +55,7 @@ inline bool ExistingDirectory(std::string filename) {
 }
 
 /// Check for a non-existing path
-inline bool NonexistentPath(std::string filename) {
+inline bool NonexistentPath(const std::string &filename) {
     struct stat buffer;
     bool exist = stat(filename.c_str(), &buffer) == 0;
     if(!exist) {
@@ -67,7 +67,7 @@ inline bool NonexistentPath(std::string filename) {
 }
 
 /// Produce a range validator function
-template <typename T> std::function<bool(std::string)> Range(T min, T max) {
+template <typename T> std::function<bool(const std::string &)> Range(T min, T max) {
     return [min, max](std::string input) {
         T val;
         detail::lexical_cast(input, val);
@@ -76,7 +76,7 @@ template <typename T> std::function<bool(std::string)> Range(T min, T max) {
 }
 
 /// Range of one value is 0 to value
-template <typename T> std::function<bool(std::string)> Range(T max) { return Range(static_cast<T>(0), max); }
+template <typename T> std::function<bool(const std::string &)> Range(T max) { return Range(static_cast<T>(0), max); }
 
 /// @}