diff --git a/sample_files/problem_setting/examples/identical_checker.cpp b/sample_files/problem_setting/examples/identical_checker.cpp
index 0afecfd..edfff56 100644
--- a/sample_files/problem_setting/examples/identical_checker.cpp
+++ b/sample_files/problem_setting/examples/identical_checker.cpp
@@ -75,10 +75,28 @@ std::string readToken(char min_char = 0, char max_char = 127) {
   return token;
 }
 
+bool validateInt(const std::string &token) {
+  if (token == "0") {
+    return true;
+  }
+  auto i = 0u;
+  if (i < token.size() && token[i] == '-') {
+    ++i;
+  }
+  if (i >= token.size() || token[i] < '1' || '9' < token[i]) {
+    return false;
+  }
+  for (i++; i < token.size(); i++) {
+    if (token[i] < '0' || '9' < token[i]) {
+      return false;
+    }
+  }
+  return true;
+}
+
 long long readInt(long long lo, long long hi) {
-  static regex_t re = regex_helpers::compile("^(0|-?[1-9][0-9]*)$");
   std::string token = readToken();
-  assertWA(regex_helpers::match(re, token));
+  assertWA(validateInt(token));
 
   long long parsedInt;
   try {
diff --git a/sample_files/problem_setting/examples/standard_interactor.cpp b/sample_files/problem_setting/examples/standard_interactor.cpp
index dbc556f..7e304b7 100644
--- a/sample_files/problem_setting/examples/standard_interactor.cpp
+++ b/sample_files/problem_setting/examples/standard_interactor.cpp
@@ -129,10 +129,28 @@ std::string readToken(char min_char = 0, char max_char = 127) {
   return token;
 }
 
+bool validateInt(const std::string &token) {
+  if (token == "0") {
+    return true;
+  }
+  auto i = 0u;
+  if (i < token.size() && token[i] == '-') {
+    ++i;
+  }
+  if (i >= token.size() || token[i] < '1' || '9' < token[i]) {
+    return false;
+  }
+  for (i++; i < token.size(); i++) {
+    if (token[i] < '0' || '9' < token[i]) {
+      return false;
+    }
+  }
+  return true;
+}
+
 long long readInt(long long lo, long long hi) {
-  static regex_t re = regex_helpers::compile("^(0|-?[1-9][0-9]*)$");
   std::string token = readToken();
-  assertWA(regex_helpers::match(re, token));
+  assertWA(validateInt(token));
 
   long long parsedInt;
   try {
diff --git a/sample_files/problem_setting/examples/validator.cpp b/sample_files/problem_setting/examples/validator.cpp
index cecc6d6..0fafe34 100644
--- a/sample_files/problem_setting/examples/validator.cpp
+++ b/sample_files/problem_setting/examples/validator.cpp
@@ -37,10 +37,28 @@ std::string readToken(char min_char = 0, char max_char = 127) {
   return token;
 }
 
+bool validateInt(const std::string &token) {
+  if (token == "0") {
+    return true;
+  }
+  auto i = 0u;
+  if (i < token.size() && token[i] == '-') {
+    ++i;
+  }
+  if (i >= token.size() || token[i] < '1' || '9' < token[i]) {
+    return false;
+  }
+  for (i++; i < token.size(); i++) {
+    if (token[i] < '0' || '9' < token[i]) {
+      return false;
+    }
+  }
+  return true;
+}
+
 long long readInt(long long lo, long long hi) {
-  static regex_t re = regex_helpers::compile("^(0|-?[1-9][0-9]*)$");
   std::string token = readToken();
-  assert(regex_helpers::match(re, token));
+  assert(validateInt(token));
 
   long long parsedInt = stoll(token); // May throw.
   assert(lo <= parsedInt && parsedInt <= hi);
diff --git a/sample_files/problem_setting/identical_checker_interactor.cpp b/sample_files/problem_setting/identical_checker_interactor.cpp
index b6f1146..20c60fd 100644
--- a/sample_files/problem_setting/identical_checker_interactor.cpp
+++ b/sample_files/problem_setting/identical_checker_interactor.cpp
@@ -87,10 +87,28 @@ std::string readLine(char min_char = 0, char max_char = 127) {
   return line;
 }
 
+bool validateInt(const std::string &token) {
+  if (token == "0") {
+    return true;
+  }
+  auto i = 0u;
+  if (i < token.size() && token[i] == '-') {
+    ++i;
+  }
+  if (i >= token.size() || token[i] < '1' || '9' < token[i]) {
+    return false;
+  }
+  for (i++; i < token.size(); i++) {
+    if (token[i] < '0' || '9' < token[i]) {
+      return false;
+    }
+  }
+  return true;
+}
+
 long long readInt(long long lo, long long hi) {
-  static regex_t re = regex_helpers::compile("^(0|-?[1-9][0-9]*)$");
   std::string token = readToken();
-  assertWA(regex_helpers::match(re, token));
+  assertWA(validateInt(token));
 
   long long parsedInt;
   try {
diff --git a/sample_files/problem_setting/standard_checker_interactor.cpp b/sample_files/problem_setting/standard_checker_interactor.cpp
index 4a20f36..4343ebd 100644
--- a/sample_files/problem_setting/standard_checker_interactor.cpp
+++ b/sample_files/problem_setting/standard_checker_interactor.cpp
@@ -130,10 +130,28 @@ std::string readToken(char min_char = 0, char max_char = 127) {
   return token;
 }
 
+bool validateInt(const std::string &token) {
+  if (token == "0") {
+    return true;
+  }
+  auto i = 0u;
+  if (i < token.size() && token[i] == '-') {
+    ++i;
+  }
+  if (i >= token.size() || token[i] < '1' || '9' < token[i]) {
+    return false;
+  }
+  for (i++; i < token.size(); i++) {
+    if (token[i] < '0' || '9' < token[i]) {
+      return false;
+    }
+  }
+  return true;
+}
+
 long long readInt(long long lo, long long hi) {
-  static regex_t re = regex_helpers::compile("^(0|-?[1-9][0-9]*)$");
   std::string token = readToken();
-  assertWA(regex_helpers::match(re, token));
+  assertWA(validateInt(token));
 
   long long parsedInt;
   try {
diff --git a/sample_files/problem_setting/validator.cpp b/sample_files/problem_setting/validator.cpp
index 75e6362..f8e613c 100644
--- a/sample_files/problem_setting/validator.cpp
+++ b/sample_files/problem_setting/validator.cpp
@@ -52,10 +52,28 @@ std::string readLine(char min_char = 0, char max_char = 127) {
   return line;
 }
 
+bool validateInt(const std::string &token) {
+  if (token == "0") {
+    return true;
+  }
+  auto i = 0u;
+  if (i < token.size() && token[i] == '-') {
+    ++i;
+  }
+  if (i >= token.size() || token[i] < '1' || '9' < token[i]) {
+    return false;
+  }
+  for (i++; i < token.size(); i++) {
+    if (token[i] < '0' || '9' < token[i]) {
+      return false;
+    }
+  }
+  return true;
+}
+
 long long readInt(long long lo, long long hi) {
-  static regex_t re = regex_helpers::compile("^(0|-?[1-9][0-9]*)$");
   std::string token = readToken();
-  assert(regex_helpers::match(re, token));
+  assert(validateInt(token));
 
   long long parsedInt = stoll(token); // May throw.
   assert(lo <= parsedInt && parsedInt <= hi);