diff --git a/headers/modsecurity/anchored_set_variable_translation_proxy.h b/headers/modsecurity/anchored_set_variable_translation_proxy.h
index f36c69b16..10f3f7f2d 100644
--- a/headers/modsecurity/anchored_set_variable_translation_proxy.h
+++ b/headers/modsecurity/anchored_set_variable_translation_proxy.h
@@ -100,13 +100,9 @@ class AnchoredSetVariableTranslationProxy {
             return nullptr;
         }
 
-        std::unique_ptr<std::string> ret(new std::string(""));
+        auto ret = std::make_unique<std::string>(l[0]->getValue());
 
-        ret->assign(l.at(0)->getValue());
-
-        while (!l.empty()) {
-            auto &a = l.back();
-            l.pop_back();
+        for(auto a : l) {
             delete a;
         }
 
diff --git a/headers/modsecurity/rules.h b/headers/modsecurity/rules.h
index 1aaa65b54..a9bfb80d3 100644
--- a/headers/modsecurity/rules.h
+++ b/headers/modsecurity/rules.h
@@ -41,9 +41,9 @@ namespace modsecurity {
 class Rules {
  public:
     void dump() const {
-        for (int j = 0; j < m_rules.size(); j++) {
-            std::cout << "    Rule ID: " << m_rules.at(j)->getReference();
-            std::cout << "--" << m_rules.at(j) << std::endl;
+        for (const auto &r : m_rules) {
+            std::cout << "    Rule ID: " << r->getReference();
+            std::cout << "--" << r << std::endl;
         }
     }
 
diff --git a/src/actions/xmlns.cc b/src/actions/xmlns.cc
index bca3b32fe..cb4044d83 100644
--- a/src/actions/xmlns.cc
+++ b/src/actions/xmlns.cc
@@ -43,7 +43,7 @@ bool XmlNS::init(std::string *error) {
         return false;
     }
 
-    if (m_href.at(0) == '\'' && m_href.size() > 3) {
+    if (m_href[0] == '\'' && m_href.size() > 3) {
         m_href.erase(0, 1);
         m_href.pop_back();
     }
diff --git a/src/operators/contains_word.cc b/src/operators/contains_word.cc
index 3de678516..e1ca9b869 100644
--- a/src/operators/contains_word.cc
+++ b/src/operators/contains_word.cc
@@ -23,13 +23,14 @@
 namespace modsecurity {
 namespace operators {
 
-bool ContainsWord::acceptableChar(const std::string& a, size_t pos) {
+inline bool ContainsWord::acceptableChar(const std::string& a, size_t pos) {
     if (a.size() - 1 < pos) {
         return false;
     }
 
-    if ((a.at(pos) >= 65 && a.at(pos) <= 90) ||
-        (a.at(pos) >= 97 && a.at(pos) <= 122)) {
+    const auto ch = a[pos];
+    if ((ch >= 65 && ch <= 90) ||
+        (ch >= 97 && ch <= 122)) {
         return false;
     }
 
diff --git a/src/operators/inspect_file.cc b/src/operators/inspect_file.cc
index 72052a32d..3796d26e6 100644
--- a/src/operators/inspect_file.cc
+++ b/src/operators/inspect_file.cc
@@ -77,7 +77,7 @@ bool InspectFile::evaluate(Transaction *transaction, const std::string &str) {
         pclose(in);
 
         res.append(s.str());
-        if (res.size() > 1 && res.at(0) != '1') {
+        if (res.size() > 1 && res[0] != '1') {
             return true; /* match */
         }
 
diff --git a/src/operators/pm.cc b/src/operators/pm.cc
index 91e4f5d7b..6a44b2860 100644
--- a/src/operators/pm.cc
+++ b/src/operators/pm.cc
@@ -32,7 +32,7 @@ static inline std::string parse_pm_content(const std::string &op_parm) {
 
     auto size = op_parm.size() - offset;
     if (size >= 2 &&
-        op_parm.at(offset) == '\"' && op_parm.back() == '\"') {
+        op_parm[offset] == '\"' && op_parm.back() == '\"') {
         offset++;
         size -= 2;
     }
diff --git a/src/operators/validate_byte_range.cc b/src/operators/validate_byte_range.cc
index 47802cef4..2553b9c1a 100644
--- a/src/operators/validate_byte_range.cc
+++ b/src/operators/validate_byte_range.cc
@@ -115,8 +115,8 @@ bool ValidateByteRange::evaluate(Transaction *transaction, RuleWithActions *rule
     bool ret = true;
 
     size_t count = 0;
-    for (int i = 0; i < input.length(); i++) {
-        int x = (unsigned char) input.at(i);
+    for (std::string::size_type i = 0; i < input.length(); i++) {
+        int x = (unsigned char) input[i];
         if (!(table[x >> 3] & (1 << (x & 0x7)))) {
             // debug(9, "Value " + std::to_string(x) + " in " +
             //     input + " ouside range: " + param);
diff --git a/src/rule_with_operator.cc b/src/rule_with_operator.cc
index e2864eb03..a3e35bf0b 100644
--- a/src/rule_with_operator.cc
+++ b/src/rule_with_operator.cc
@@ -194,8 +194,7 @@ inline void RuleWithOperator::getFinalVars(variables::Variables *vars,
         vars->push_back(variable);
     }
 
-    for (int i = 0; i < addition.size(); i++) {
-        Variable *variable = addition.at(i);
+    for (auto *variable : addition) {
         vars->push_back(variable);
     }
 }
diff --git a/src/rules_set_phases.cc b/src/rules_set_phases.cc
index 1d93330ca..a78193049 100644
--- a/src/rules_set_phases.cc
+++ b/src/rules_set_phases.cc
@@ -45,8 +45,8 @@ int RulesSetPhases::append(RulesSetPhases *from, std::ostringstream *err) {
 
     for (int i = 0; i < modsecurity::Phases::NUMBER_OF_PHASES; i++) {
         v.reserve(m_rulesAtPhase[i].size());
-        for (size_t z = 0; z < m_rulesAtPhase[i].size(); z++) {
-            RuleWithOperator *rule_ckc = dynamic_cast<RuleWithOperator *>(m_rulesAtPhase[i].at(z).get());
+        for (const auto &r : m_rulesAtPhase[i].m_rules) {
+            const auto *rule_ckc = dynamic_cast<const RuleWithOperator *>(r.get());
             if (!rule_ckc) {
                 continue;
             }
diff --git a/src/transaction.cc b/src/transaction.cc
index 189225231..f5ded5246 100644
--- a/src/transaction.cc
+++ b/src/transaction.cc
@@ -405,7 +405,7 @@ int Transaction::processURI(const char *uri, const char *method,
 
     std::string parsedURI = m_uri_decoded;
     // The more popular case is without domain
-    if (!m_uri_decoded.empty() && m_uri_decoded.at(0) != '/') {
+    if (!m_uri_decoded.empty() && m_uri_decoded[0] != '/') {
         bool fullDomain = true;
         size_t scheme = m_uri_decoded.find(":")+1;
         if (scheme == std::string::npos) {
@@ -540,7 +540,7 @@ int Transaction::addRequestHeader(const std::string& key,
             }
 
             // ltrim the key - following the modsec v2 way
-            while (ckey.empty() == false && isspace(ckey.at(0))) {
+            while (ckey.empty() == false && isspace(ckey[0])) {
                 ckey.erase(0, 1);
                 localOffset++;
             }
diff --git a/src/utils/string.h b/src/utils/string.h
index f5e52ff06..f5794686b 100644
--- a/src/utils/string.h
+++ b/src/utils/string.h
@@ -107,12 +107,12 @@ inline std::string toHexIfNeeded(const std::string &str, bool escape_spec = fals
     // spec chars: '"' (quotation mark, ascii 34), '\' (backslash, ascii 92)
     std::stringstream res;
 
-    for (int i = 0; i < str.size(); i++) {
-        int c = (unsigned char)str.at(i);
+    for (const auto ch : str) {
+        int c = (unsigned char)ch;
         if (c < 32 || c > 126 || (escape_spec == true && (c == 34 || c == 92))) {
             res << "\\x" << std::setw(2) << std::setfill('0') << std::hex << c;
         } else {
-            res << str.at(i);
+            res << ch;
         }
     }
 
@@ -177,10 +177,10 @@ inline void replaceAll(std::string &str, std::string_view from,
 
 
 inline std::string removeWhiteSpacesIfNeeded(std::string a) {
-    while (a.size() > 1 && a.at(0) == ' ') {
+    while (a.size() > 1 && a.front() == ' ') {
         a.erase(0, 1);
     }
-    while (a.size() > 1 && a.at(a.length()-1) == ' ') {
+    while (a.size() > 1 && a.back() == ' ') {
         a.pop_back();
     }
     return a;
@@ -188,11 +188,11 @@ inline std::string removeWhiteSpacesIfNeeded(std::string a) {
 
 
 inline std::string removeBracketsIfNeeded(std::string a) {
-    if (a.length() > 1 && a.at(0) == '"' && a.at(a.length()-1) == '"') {
+    if (a.length() > 1 && a.front() == '"' && a.back() == '"') {
         a.pop_back();
         a.erase(0, 1);
     }
-    if (a.length() > 1 && a.at(0) == '\'' && a.at(a.length()-1) == '\'') {
+    if (a.length() > 1 && a.front() == '\'' && a.back() == '\'') {
         a.pop_back();
         a.erase(0, 1);
     }