diff --git a/headers/modsecurity/actions/action.h b/headers/modsecurity/actions/action.h
index 03ab5c7d..9fb4398b 100644
--- a/headers/modsecurity/actions/action.h
+++ b/headers/modsecurity/actions/action.h
@@ -57,7 +57,7 @@ class Action {
 
     virtual ~Action() { }
 
-    virtual std::string evaluate(std::string exp,
+    virtual std::string evaluate(const std::string &exp,
         Transaction *transaction);
     virtual bool evaluate(Rule *rule, Transaction *transaction);
     virtual bool evaluate(Rule *rule, Transaction *transaction,
diff --git a/src/actions/action.cc b/src/actions/action.cc
index 4e781458..e07a3b0a 100644
--- a/src/actions/action.cc
+++ b/src/actions/action.cc
@@ -45,7 +45,7 @@ namespace modsecurity {
 namespace actions {
 
 
-std::string Action::evaluate(std::string value,
+std::string Action::evaluate(const std::string &value,
     Transaction *transaction) {
     return value;
 }
diff --git a/src/actions/transformations/base64_decode.cc b/src/actions/transformations/base64_decode.cc
index 9b81e4e9..e72fefea 100644
--- a/src/actions/transformations/base64_decode.cc
+++ b/src/actions/transformations/base64_decode.cc
@@ -32,7 +32,7 @@ namespace actions {
 namespace transformations {
 
 
-std::string Base64Decode::evaluate(std::string value,
+std::string Base64Decode::evaluate(const std::string &value,
     Transaction *transaction) {
     std::string ret = Utils::Base64::decode(value);
 
diff --git a/src/actions/transformations/base64_decode.h b/src/actions/transformations/base64_decode.h
index a9dbc4cf..6676a99e 100644
--- a/src/actions/transformations/base64_decode.h
+++ b/src/actions/transformations/base64_decode.h
@@ -32,7 +32,7 @@ class Base64Decode : public Transformation {
  public:
     explicit Base64Decode(const std::string &action)  : Transformation(action) { }
 
-    std::string evaluate(std::string exp,
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 };
 
diff --git a/src/actions/transformations/base64_decode_ext.cc b/src/actions/transformations/base64_decode_ext.cc
index 6f4f524d..3ea9887c 100644
--- a/src/actions/transformations/base64_decode_ext.cc
+++ b/src/actions/transformations/base64_decode_ext.cc
@@ -32,7 +32,7 @@ namespace actions {
 namespace transformations {
 
 
-std::string Base64DecodeExt::evaluate(std::string value,
+std::string Base64DecodeExt::evaluate(const std::string &value,
     Transaction *transaction) {
     std::string ret = Utils::Base64::decode_forgiven(value);
 
diff --git a/src/actions/transformations/base64_decode_ext.h b/src/actions/transformations/base64_decode_ext.h
index a8793c0f..595e31f2 100644
--- a/src/actions/transformations/base64_decode_ext.h
+++ b/src/actions/transformations/base64_decode_ext.h
@@ -32,7 +32,7 @@ class Base64DecodeExt : public Transformation {
  public:
     explicit Base64DecodeExt(const std::string &action)  : Transformation(action) { }
 
-    std::string evaluate(std::string exp,
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 };
 
diff --git a/src/actions/transformations/base64_encode.cc b/src/actions/transformations/base64_encode.cc
index 7f256d63..ffc2465a 100644
--- a/src/actions/transformations/base64_encode.cc
+++ b/src/actions/transformations/base64_encode.cc
@@ -32,7 +32,7 @@ namespace actions {
 namespace transformations {
 
 
-std::string Base64Encode::evaluate(std::string value,
+std::string Base64Encode::evaluate(const std::string &value,
     Transaction *transaction) {
     std::string ret = Utils::Base64::encode(value);
 
diff --git a/src/actions/transformations/base64_encode.h b/src/actions/transformations/base64_encode.h
index 8db2b817..a5dfd0ba 100644
--- a/src/actions/transformations/base64_encode.h
+++ b/src/actions/transformations/base64_encode.h
@@ -32,7 +32,7 @@ class Base64Encode : public Transformation {
  public:
     explicit Base64Encode(const std::string &action)  : Transformation(action) { }
 
-    std::string evaluate(std::string exp,
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 };
 
diff --git a/src/actions/transformations/cmd_line.cc b/src/actions/transformations/cmd_line.cc
index da8a80c1..8fd73e81 100644
--- a/src/actions/transformations/cmd_line.cc
+++ b/src/actions/transformations/cmd_line.cc
@@ -31,7 +31,7 @@ namespace actions {
 namespace transformations {
 
 
-std::string CmdLine::evaluate(std::string value,
+std::string CmdLine::evaluate(const std::string &value,
     Transaction *transaction) {
     std::string ret;
     int space = 0;
diff --git a/src/actions/transformations/cmd_line.h b/src/actions/transformations/cmd_line.h
index db357407..81baf848 100644
--- a/src/actions/transformations/cmd_line.h
+++ b/src/actions/transformations/cmd_line.h
@@ -33,7 +33,7 @@ class CmdLine : public Transformation {
     explicit CmdLine(const std::string &action) 
         : Transformation(action) { }
 
-    std::string evaluate(std::string exp,
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 };
 
diff --git a/src/actions/transformations/compress_whitespace.cc b/src/actions/transformations/compress_whitespace.cc
index 7b110519..3a0518d7 100644
--- a/src/actions/transformations/compress_whitespace.cc
+++ b/src/actions/transformations/compress_whitespace.cc
@@ -35,7 +35,7 @@ CompressWhitespace::CompressWhitespace(const std::string &action)
     this->action_kind = 1;
 }
 
-std::string CompressWhitespace::evaluate(std::string value,
+std::string CompressWhitespace::evaluate(const std::string &value,
     Transaction *transaction) {
 
     std::string a;
diff --git a/src/actions/transformations/compress_whitespace.h b/src/actions/transformations/compress_whitespace.h
index f38094fa..d8b19370 100644
--- a/src/actions/transformations/compress_whitespace.h
+++ b/src/actions/transformations/compress_whitespace.h
@@ -30,8 +30,10 @@ namespace transformations {
 
 class CompressWhitespace : public Transformation {
  public:
+
     explicit CompressWhitespace(const std::string &action) ;
-    std::string evaluate(std::string exp,
+
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 };
 
diff --git a/src/actions/transformations/css_decode.cc b/src/actions/transformations/css_decode.cc
index cadf68cc..4b23618f 100644
--- a/src/actions/transformations/css_decode.cc
+++ b/src/actions/transformations/css_decode.cc
@@ -34,7 +34,7 @@ namespace actions {
 namespace transformations {
 
 
-std::string CssDecode::evaluate(std::string value,
+std::string CssDecode::evaluate(const std::string &value,
     Transaction *transaction) {
 
     char *tmp = reinterpret_cast<char *>(
diff --git a/src/actions/transformations/css_decode.h b/src/actions/transformations/css_decode.h
index cab69740..5d41e04d 100644
--- a/src/actions/transformations/css_decode.h
+++ b/src/actions/transformations/css_decode.h
@@ -33,7 +33,7 @@ class CssDecode : public Transformation {
  public:
     explicit CssDecode(const std::string &action) 
         : Transformation(action) { }
-    std::string evaluate(std::string exp,
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 
     static int css_decode_inplace(unsigned char *input, int64_t input_len);
diff --git a/src/actions/transformations/escape_seq_decode.cc b/src/actions/transformations/escape_seq_decode.cc
index d8fbea5a..e32a42d0 100644
--- a/src/actions/transformations/escape_seq_decode.cc
+++ b/src/actions/transformations/escape_seq_decode.cc
@@ -140,7 +140,7 @@ int EscapeSeqDecode::ansi_c_sequences_decode_inplace(unsigned char *input,
 }
 
 
-std::string EscapeSeqDecode::evaluate(std::string value,
+std::string EscapeSeqDecode::evaluate(const std::string &value,
     Transaction *transaction) {
 
     unsigned char *tmp = (unsigned char *) malloc(sizeof(char)
diff --git a/src/actions/transformations/escape_seq_decode.h b/src/actions/transformations/escape_seq_decode.h
index 98370a02..654548aa 100644
--- a/src/actions/transformations/escape_seq_decode.h
+++ b/src/actions/transformations/escape_seq_decode.h
@@ -30,8 +30,10 @@ namespace transformations {
 
 class EscapeSeqDecode : public Transformation {
  public:
+
     explicit EscapeSeqDecode(const std::string &action) ;
-    std::string evaluate(std::string exp,
+
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
     int ansi_c_sequences_decode_inplace(unsigned char *input, int input_len);
 };
diff --git a/src/actions/transformations/hex_decode.cc b/src/actions/transformations/hex_decode.cc
index 44e30f5f..bc72228f 100644
--- a/src/actions/transformations/hex_decode.cc
+++ b/src/actions/transformations/hex_decode.cc
@@ -32,7 +32,7 @@ namespace actions {
 namespace transformations {
 
 
-std::string HexDecode::evaluate(std::string value,
+std::string HexDecode::evaluate(const std::string &value,
     Transaction *transaction) {
     std::string ret;
     unsigned char *input;
diff --git a/src/actions/transformations/hex_decode.h b/src/actions/transformations/hex_decode.h
index bbf49b2a..8f8aa2eb 100644
--- a/src/actions/transformations/hex_decode.h
+++ b/src/actions/transformations/hex_decode.h
@@ -32,7 +32,7 @@ class HexDecode : public Transformation {
  public:
     explicit HexDecode(const std::string &action)  : Transformation(action) { }
 
-    std::string evaluate(std::string exp,
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 
     static int inplace(unsigned char *data, int len);
diff --git a/src/actions/transformations/hex_encode.cc b/src/actions/transformations/hex_encode.cc
index 4b01230c..912c6f27 100644
--- a/src/actions/transformations/hex_encode.cc
+++ b/src/actions/transformations/hex_encode.cc
@@ -36,7 +36,7 @@ HexEncode::HexEncode(const std::string &action)
     this->action_kind = 1;
 }
 
-std::string HexEncode::evaluate(std::string value,
+std::string HexEncode::evaluate(const std::string &value,
     Transaction *transaction) {
 
     std::stringstream result;
diff --git a/src/actions/transformations/hex_encode.h b/src/actions/transformations/hex_encode.h
index d785c0b1..b76e2672 100644
--- a/src/actions/transformations/hex_encode.h
+++ b/src/actions/transformations/hex_encode.h
@@ -30,8 +30,10 @@ namespace transformations {
 
 class HexEncode : public Transformation {
  public:
-    explicit HexEncode(const std::string &action) ;
-    std::string evaluate(std::string exp,
+
+    explicit HexEncode(const std::string &action);
+
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 };
 
diff --git a/src/actions/transformations/html_entity_decode.cc b/src/actions/transformations/html_entity_decode.cc
index 0a3e996d..48257e76 100644
--- a/src/actions/transformations/html_entity_decode.cc
+++ b/src/actions/transformations/html_entity_decode.cc
@@ -33,7 +33,7 @@ namespace actions {
 namespace transformations {
 
 
-std::string HtmlEntityDecode::evaluate(std::string value,
+std::string HtmlEntityDecode::evaluate(const std::string &value,
     Transaction *transaction) {
     std::string ret;
     unsigned char *input;
diff --git a/src/actions/transformations/html_entity_decode.h b/src/actions/transformations/html_entity_decode.h
index 1c8c620d..eaf5ae0b 100644
--- a/src/actions/transformations/html_entity_decode.h
+++ b/src/actions/transformations/html_entity_decode.h
@@ -36,7 +36,7 @@ class HtmlEntityDecode : public Transformation {
     explicit HtmlEntityDecode(const std::string &action) 
         : Transformation(action) { }
 
-    std::string evaluate(std::string exp,
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 
     static int inplace(unsigned char *input, uint64_t input_len);
diff --git a/src/actions/transformations/js_decode.cc b/src/actions/transformations/js_decode.cc
index 6c01e7fd..89f33b9f 100644
--- a/src/actions/transformations/js_decode.cc
+++ b/src/actions/transformations/js_decode.cc
@@ -34,7 +34,7 @@ namespace actions {
 namespace transformations {
 
 
-std::string JsDecode::evaluate(std::string value,
+std::string JsDecode::evaluate(const std::string &value,
     Transaction *transaction) {
     std::string ret;
     unsigned char *input;
diff --git a/src/actions/transformations/js_decode.h b/src/actions/transformations/js_decode.h
index 1689501c..e680aebc 100644
--- a/src/actions/transformations/js_decode.h
+++ b/src/actions/transformations/js_decode.h
@@ -33,7 +33,7 @@ class JsDecode : public Transformation {
     explicit JsDecode(const std::string &action) 
         : Transformation(action) { }
 
-    std::string evaluate(std::string exp,
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
     static int inplace(unsigned char *input, uint64_t input_len);
 };
diff --git a/src/actions/transformations/length.cc b/src/actions/transformations/length.cc
index 8a902427..e27cd41b 100644
--- a/src/actions/transformations/length.cc
+++ b/src/actions/transformations/length.cc
@@ -35,7 +35,7 @@ Length::Length(const std::string &action)
     this->action_kind = 1;
 }
 
-std::string Length::evaluate(std::string value,
+std::string Length::evaluate(const std::string &value,
     Transaction *transaction) {
 
     return std::to_string(value.size());
diff --git a/src/actions/transformations/length.h b/src/actions/transformations/length.h
index 07606469..7974b5c1 100644
--- a/src/actions/transformations/length.h
+++ b/src/actions/transformations/length.h
@@ -30,8 +30,10 @@ namespace transformations {
 
 class Length : public Transformation {
  public:
-    explicit Length(const std::string &action) ;
-    std::string evaluate(std::string exp,
+
+    explicit Length(const std::string &action);
+
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 };
 
diff --git a/src/actions/transformations/lower_case.cc b/src/actions/transformations/lower_case.cc
index baabc77c..fcf3ba79 100644
--- a/src/actions/transformations/lower_case.cc
+++ b/src/actions/transformations/lower_case.cc
@@ -31,9 +31,10 @@ LowerCase::LowerCase(const std::string &a)
     : Transformation(a) {
 }
 
-std::string LowerCase::evaluate(std::string value,
+std::string LowerCase::evaluate(const std::string &val,
     Transaction *transaction) {
     std::locale loc;
+    std::string value(val);
 
     for (std::string::size_type i=0; i < value.length(); ++i) {
         value[i] = std::tolower(value[i], loc);
diff --git a/src/actions/transformations/lower_case.h b/src/actions/transformations/lower_case.h
index 41ddf795..12eee321 100644
--- a/src/actions/transformations/lower_case.h
+++ b/src/actions/transformations/lower_case.h
@@ -32,8 +32,8 @@ namespace transformations {
 
 class LowerCase : public Transformation {
  public:
-    explicit LowerCase(const std::string &action) ;
-    std::string evaluate(std::string exp,
+    explicit LowerCase(const std::string &action);
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 };
 
diff --git a/src/actions/transformations/md5.cc b/src/actions/transformations/md5.cc
index 17e15c07..8becdb26 100644
--- a/src/actions/transformations/md5.cc
+++ b/src/actions/transformations/md5.cc
@@ -31,9 +31,9 @@ namespace actions {
 namespace transformations {
 
 
-std::string Md5::evaluate(std::string value,
+std::string Md5::evaluate(const std::string &value,
     Transaction *transaction) {
-    std::string ret =  Utils::Md5::digest(value);
+    std::string ret = Utils::Md5::digest(value);
 
     return ret;
 }
diff --git a/src/actions/transformations/md5.h b/src/actions/transformations/md5.h
index 7e04ed15..1742b8b5 100644
--- a/src/actions/transformations/md5.h
+++ b/src/actions/transformations/md5.h
@@ -32,7 +32,7 @@ class Md5 : public Transformation {
  public:
     explicit Md5(const std::string &action)  : Transformation(action) { }
 
-    std::string evaluate(std::string exp,
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 };
 
diff --git a/src/actions/transformations/none.cc b/src/actions/transformations/none.cc
index 67229e7a..79ab0b84 100644
--- a/src/actions/transformations/none.cc
+++ b/src/actions/transformations/none.cc
@@ -31,7 +31,7 @@ namespace actions {
 namespace transformations {
 
 
-std::string None::evaluate(std::string value,
+std::string None::evaluate(const std::string &value,
     Transaction *transaction) {
     return value;
 }
diff --git a/src/actions/transformations/none.h b/src/actions/transformations/none.h
index 7cee4b42..00bf37a7 100644
--- a/src/actions/transformations/none.h
+++ b/src/actions/transformations/none.h
@@ -34,7 +34,7 @@ class None : public Transformation {
         : Transformation(action)
         { m_isNone = true; }
 
-    std::string evaluate(std::string exp,
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 };
 
diff --git a/src/actions/transformations/normalise_path.cc b/src/actions/transformations/normalise_path.cc
index 3c184976..ad76a22a 100644
--- a/src/actions/transformations/normalise_path.cc
+++ b/src/actions/transformations/normalise_path.cc
@@ -37,7 +37,7 @@ NormalisePath::NormalisePath(const std::string &action)
     this->action_kind = 1;
 }
 
-std::string NormalisePath::evaluate(std::string value,
+std::string NormalisePath::evaluate(const std::string &value,
     Transaction *transaction) {
     int changed = 0;
 
diff --git a/src/actions/transformations/normalise_path.h b/src/actions/transformations/normalise_path.h
index e41a915c..3c98d7f9 100644
--- a/src/actions/transformations/normalise_path.h
+++ b/src/actions/transformations/normalise_path.h
@@ -30,8 +30,10 @@ namespace transformations {
 
 class NormalisePath : public Transformation {
  public:
-    explicit NormalisePath(const std::string &action) ;
-    std::string evaluate(std::string exp,
+
+    explicit NormalisePath(const std::string &action);
+
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 
     static int normalize_path_inplace(unsigned char *input, int input_len,
diff --git a/src/actions/transformations/normalise_path_win.cc b/src/actions/transformations/normalise_path_win.cc
index d4198acd..8970e8b9 100644
--- a/src/actions/transformations/normalise_path_win.cc
+++ b/src/actions/transformations/normalise_path_win.cc
@@ -34,7 +34,7 @@ namespace actions {
 namespace transformations {
 
 
-std::string NormalisePathWin::evaluate(std::string value,
+std::string NormalisePathWin::evaluate(const std::string &value,
     Transaction *transaction) {
     int changed;
 
diff --git a/src/actions/transformations/normalise_path_win.h b/src/actions/transformations/normalise_path_win.h
index 360bc832..61ce0ca0 100644
--- a/src/actions/transformations/normalise_path_win.h
+++ b/src/actions/transformations/normalise_path_win.h
@@ -33,7 +33,7 @@ class NormalisePathWin : public Transformation {
     explicit NormalisePathWin(const std::string &action) 
         : Transformation(action) { }
 
-    std::string evaluate(std::string exp,
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 };
 
diff --git a/src/actions/transformations/parity_even_7bit.cc b/src/actions/transformations/parity_even_7bit.cc
index 1da74b8c..357fe759 100644
--- a/src/actions/transformations/parity_even_7bit.cc
+++ b/src/actions/transformations/parity_even_7bit.cc
@@ -32,7 +32,7 @@ namespace actions {
 namespace transformations {
 
 
-std::string ParityEven7bit::evaluate(std::string value,
+std::string ParityEven7bit::evaluate(const std::string &value,
     Transaction *transaction) {
     std::string ret;
     unsigned char *input;
diff --git a/src/actions/transformations/parity_even_7bit.h b/src/actions/transformations/parity_even_7bit.h
index 5996f3d8..7ec5f2aa 100644
--- a/src/actions/transformations/parity_even_7bit.h
+++ b/src/actions/transformations/parity_even_7bit.h
@@ -32,7 +32,7 @@ class ParityEven7bit : public Transformation {
  public:
     explicit ParityEven7bit(const std::string &action)  : Transformation(action) { }
 
-    std::string evaluate(std::string exp, Transaction *transaction) override;
+    std::string evaluate(const std::string &exp, Transaction *transaction) override;
     static bool inplace(unsigned char *input, uint64_t input_len);
 };
 
diff --git a/src/actions/transformations/parity_odd_7bit.cc b/src/actions/transformations/parity_odd_7bit.cc
index 28447291..fbd6c8fc 100644
--- a/src/actions/transformations/parity_odd_7bit.cc
+++ b/src/actions/transformations/parity_odd_7bit.cc
@@ -32,7 +32,7 @@ namespace actions {
 namespace transformations {
 
 
-std::string ParityOdd7bit::evaluate(std::string value,
+std::string ParityOdd7bit::evaluate(const std::string &value,
     Transaction *transaction) {
     std::string ret;
     unsigned char *input;
diff --git a/src/actions/transformations/parity_odd_7bit.h b/src/actions/transformations/parity_odd_7bit.h
index c5e74e73..9af53722 100644
--- a/src/actions/transformations/parity_odd_7bit.h
+++ b/src/actions/transformations/parity_odd_7bit.h
@@ -32,7 +32,7 @@ class ParityOdd7bit : public Transformation {
  public:
     explicit ParityOdd7bit(const std::string &action)  : Transformation(action) { }
 
-    std::string evaluate(std::string exp, Transaction *transaction) override;
+    std::string evaluate(const std::string &exp, Transaction *transaction) override;
     static bool inplace(unsigned char *input, uint64_t input_len);
 };
 
diff --git a/src/actions/transformations/parity_zero_7bit.cc b/src/actions/transformations/parity_zero_7bit.cc
index 3ab6987c..93a0f974 100644
--- a/src/actions/transformations/parity_zero_7bit.cc
+++ b/src/actions/transformations/parity_zero_7bit.cc
@@ -32,7 +32,7 @@ namespace actions {
 namespace transformations {
 
 
-std::string ParityZero7bit::evaluate(std::string value,
+std::string ParityZero7bit::evaluate(const std::string &value,
     Transaction *transaction) {
     std::string ret;
     unsigned char *input;
diff --git a/src/actions/transformations/parity_zero_7bit.h b/src/actions/transformations/parity_zero_7bit.h
index 6c3897f9..5e30dd5d 100644
--- a/src/actions/transformations/parity_zero_7bit.h
+++ b/src/actions/transformations/parity_zero_7bit.h
@@ -32,7 +32,7 @@ class ParityZero7bit : public Transformation {
  public:
     explicit ParityZero7bit(const std::string &action)  : Transformation(action) { }
 
-    std::string evaluate(std::string exp, Transaction *transaction) override;
+    std::string evaluate(const std::string &exp, Transaction *transaction) override;
     static bool inplace(unsigned char *input, uint64_t input_len);
 };
 
diff --git a/src/actions/transformations/remove_comments.cc b/src/actions/transformations/remove_comments.cc
index 5d2a9774..4bce61b5 100644
--- a/src/actions/transformations/remove_comments.cc
+++ b/src/actions/transformations/remove_comments.cc
@@ -32,7 +32,7 @@ namespace actions {
 namespace transformations {
 
 
-std::string RemoveComments::evaluate(std::string value,
+std::string RemoveComments::evaluate(const std::string &value,
     Transaction *transaction) {
     std::string ret;
     unsigned char *input;
diff --git a/src/actions/transformations/remove_comments.h b/src/actions/transformations/remove_comments.h
index 711ca9ef..46ad2a7e 100644
--- a/src/actions/transformations/remove_comments.h
+++ b/src/actions/transformations/remove_comments.h
@@ -33,7 +33,7 @@ class RemoveComments : public Transformation {
  public:
     explicit RemoveComments(const std::string &action)  : Transformation(action) { }
 
-    std::string evaluate(std::string exp,
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 };
 
diff --git a/src/actions/transformations/remove_comments_char.cc b/src/actions/transformations/remove_comments_char.cc
index 010d7aa3..68bd28c3 100644
--- a/src/actions/transformations/remove_comments_char.cc
+++ b/src/actions/transformations/remove_comments_char.cc
@@ -35,9 +35,10 @@ RemoveCommentsChar::RemoveCommentsChar(const std::string &action)
     this->action_kind = 1;
 }
 
-std::string RemoveCommentsChar::evaluate(std::string value,
+std::string RemoveCommentsChar::evaluate(const std::string &val,
     Transaction *transaction) {
     int64_t i;
+    std::string value(val);
 
     i = 0;
     while (i < value.size()) {
diff --git a/src/actions/transformations/remove_comments_char.h b/src/actions/transformations/remove_comments_char.h
index f5f60467..03091ceb 100644
--- a/src/actions/transformations/remove_comments_char.h
+++ b/src/actions/transformations/remove_comments_char.h
@@ -30,8 +30,9 @@ namespace transformations {
 
 class RemoveCommentsChar : public Transformation {
  public:
-    explicit RemoveCommentsChar(const std::string &action) ;
-    std::string evaluate(std::string exp,
+    explicit RemoveCommentsChar(const std::string &action);
+
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 };
 
diff --git a/src/actions/transformations/remove_nulls.cc b/src/actions/transformations/remove_nulls.cc
index d419c81b..991956d0 100644
--- a/src/actions/transformations/remove_nulls.cc
+++ b/src/actions/transformations/remove_nulls.cc
@@ -33,9 +33,10 @@ namespace actions {
 namespace transformations {
 
 
-std::string RemoveNulls::evaluate(std::string value,
+std::string RemoveNulls::evaluate(const std::string &val,
     Transaction *transaction) {
     int64_t i;
+    std::string value(val);
 
     i = 0;
     while (i < value.size()) {
diff --git a/src/actions/transformations/remove_nulls.h b/src/actions/transformations/remove_nulls.h
index 20c0e9db..6d5a0820 100644
--- a/src/actions/transformations/remove_nulls.h
+++ b/src/actions/transformations/remove_nulls.h
@@ -33,7 +33,7 @@ class RemoveNulls : public Transformation {
     explicit RemoveNulls(const std::string &action) 
         : Transformation(action) { }
 
-    std::string evaluate(std::string exp,
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 };
 
diff --git a/src/actions/transformations/remove_whitespace.cc b/src/actions/transformations/remove_whitespace.cc
index b5f7d5b7..a3687cc3 100644
--- a/src/actions/transformations/remove_whitespace.cc
+++ b/src/actions/transformations/remove_whitespace.cc
@@ -36,8 +36,9 @@ RemoveWhitespace::RemoveWhitespace(const std::string &action)
     this->action_kind = 1;
 }
 
-std::string RemoveWhitespace::evaluate(std::string value,
+std::string RemoveWhitespace::evaluate(const std::string &val,
     Transaction *transaction) {
+    std::string value(val);
 
     int64_t i = 0;
 
diff --git a/src/actions/transformations/remove_whitespace.h b/src/actions/transformations/remove_whitespace.h
index 746e1556..c2968ddc 100644
--- a/src/actions/transformations/remove_whitespace.h
+++ b/src/actions/transformations/remove_whitespace.h
@@ -30,8 +30,9 @@ namespace transformations {
 
 class RemoveWhitespace : public Transformation {
  public:
-    explicit RemoveWhitespace(const std::string &action) ;
-    std::string evaluate(std::string exp,
+    explicit RemoveWhitespace(const std::string &action);
+
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 };
 
diff --git a/src/actions/transformations/replace_comments.cc b/src/actions/transformations/replace_comments.cc
index 48a40fbc..6f0250cb 100644
--- a/src/actions/transformations/replace_comments.cc
+++ b/src/actions/transformations/replace_comments.cc
@@ -36,7 +36,7 @@ ReplaceComments::ReplaceComments(const std::string &action)
     this->action_kind = 1;
 }
 
-std::string ReplaceComments::evaluate(std::string value,
+std::string ReplaceComments::evaluate(const std::string &value,
     Transaction *transaction) {
     uint64_t i, j, incomment;
 
diff --git a/src/actions/transformations/replace_comments.h b/src/actions/transformations/replace_comments.h
index 24648160..a2c06a4c 100644
--- a/src/actions/transformations/replace_comments.h
+++ b/src/actions/transformations/replace_comments.h
@@ -30,8 +30,10 @@ namespace transformations {
 
 class ReplaceComments : public Transformation {
  public:
+
     explicit ReplaceComments(const std::string &action) ;
-    std::string evaluate(std::string exp,
+
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 };
 
diff --git a/src/actions/transformations/replace_nulls.cc b/src/actions/transformations/replace_nulls.cc
index df59300d..7f20d0da 100644
--- a/src/actions/transformations/replace_nulls.cc
+++ b/src/actions/transformations/replace_nulls.cc
@@ -35,9 +35,10 @@ ReplaceNulls::ReplaceNulls(const std::string &action)
     this->action_kind = 1;
 }
 
-std::string ReplaceNulls::evaluate(std::string value,
+std::string ReplaceNulls::evaluate(const std::string &val,
     Transaction *transaction) {
     int64_t i;
+    std::string value(val);
 
     i = 0;
     while (i < value.size()) {
diff --git a/src/actions/transformations/replace_nulls.h b/src/actions/transformations/replace_nulls.h
index 87155b86..7af92643 100644
--- a/src/actions/transformations/replace_nulls.h
+++ b/src/actions/transformations/replace_nulls.h
@@ -30,8 +30,10 @@ namespace transformations {
 
 class ReplaceNulls : public Transformation {
  public:
+
     explicit ReplaceNulls(const std::string &action) ;
-    std::string evaluate(std::string exp,
+
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 };
 
diff --git a/src/actions/transformations/sha1.cc b/src/actions/transformations/sha1.cc
index 3cb69dee..0c30512a 100644
--- a/src/actions/transformations/sha1.cc
+++ b/src/actions/transformations/sha1.cc
@@ -36,7 +36,7 @@ Sha1::Sha1(const std::string &action)
     this->action_kind = 1;
 }
 
-std::string Sha1::evaluate(std::string value,
+std::string Sha1::evaluate(const std::string &value,
     Transaction *transaction) {
 
     return Utils::Sha1::digest(value);
diff --git a/src/actions/transformations/sha1.h b/src/actions/transformations/sha1.h
index 318128b0..87e5aaa1 100644
--- a/src/actions/transformations/sha1.h
+++ b/src/actions/transformations/sha1.h
@@ -31,7 +31,7 @@ namespace transformations {
 class Sha1 : public Transformation {
  public:
     explicit Sha1(const std::string &action) ;
-    std::string evaluate(std::string exp,
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 };
 
diff --git a/src/actions/transformations/sql_hex_decode.cc b/src/actions/transformations/sql_hex_decode.cc
index dcf872b7..b33deac7 100644
--- a/src/actions/transformations/sql_hex_decode.cc
+++ b/src/actions/transformations/sql_hex_decode.cc
@@ -41,7 +41,7 @@ namespace transformations {
 #define ISODIGIT(X) ((X >= '0') && (X <= '7'))
 #endif
 
-std::string SqlHexDecode::evaluate(std::string value,
+std::string SqlHexDecode::evaluate(const std::string &value,
     Transaction *transaction) {
     std::string ret;
     unsigned char *input;
diff --git a/src/actions/transformations/sql_hex_decode.h b/src/actions/transformations/sql_hex_decode.h
index 2d04d90e..4ef5a63e 100644
--- a/src/actions/transformations/sql_hex_decode.h
+++ b/src/actions/transformations/sql_hex_decode.h
@@ -32,7 +32,7 @@ class SqlHexDecode : public Transformation {
  public:
     explicit SqlHexDecode(const std::string &action)  : Transformation(action) { }
 
-    std::string evaluate(std::string exp,
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 
     static int inplace(unsigned char *data, int len);
diff --git a/src/actions/transformations/transformation.cc b/src/actions/transformations/transformation.cc
index 71ea175f..fae80ee6 100644
--- a/src/actions/transformations/transformation.cc
+++ b/src/actions/transformations/transformation.cc
@@ -69,7 +69,7 @@ namespace actions {
 namespace transformations {
 
 
-std::string Transformation::evaluate(std::string value,
+std::string Transformation::evaluate(const std::string &value,
     Transaction *transaction) {
     return value;
 }
diff --git a/src/actions/transformations/transformation.h b/src/actions/transformations/transformation.h
index 77baeff2..57f20416 100644
--- a/src/actions/transformations/transformation.h
+++ b/src/actions/transformations/transformation.h
@@ -35,7 +35,7 @@ class Transformation : public Action {
     explicit Transformation(const std::string& _action, int kind)
         : Action(_action, kind) { }
 
-    std::string evaluate(std::string exp,
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 
     static Transformation* instantiate(std::string a);
diff --git a/src/actions/transformations/trim.cc b/src/actions/transformations/trim.cc
index 2c8f158b..4954237a 100644
--- a/src/actions/transformations/trim.cc
+++ b/src/actions/transformations/trim.cc
@@ -57,8 +57,9 @@ Trim::Trim(const std::string &action)
 
 
 std::string
-Trim::evaluate(std::string value,
+Trim::evaluate(const std::string &val,
     Transaction *transaction) {
+    std::string value(val);
     return *this->trim(&value);
 }
 
diff --git a/src/actions/transformations/trim.h b/src/actions/transformations/trim.h
index dd215ec0..4deebb4f 100644
--- a/src/actions/transformations/trim.h
+++ b/src/actions/transformations/trim.h
@@ -30,8 +30,10 @@ namespace transformations {
 
 class Trim : public Transformation {
  public:
+
     explicit Trim(const std::string &action) ;
-    std::string evaluate(std::string exp,
+
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 
     std::string *ltrim(std::string *s);
diff --git a/src/actions/transformations/trim_left.cc b/src/actions/transformations/trim_left.cc
index eff0167d..9116e3c8 100644
--- a/src/actions/transformations/trim_left.cc
+++ b/src/actions/transformations/trim_left.cc
@@ -38,8 +38,9 @@ TrimLeft::TrimLeft(const std::string &action)
     this->action_kind = 1;
 }
 
-std::string TrimLeft::evaluate(std::string value,
+std::string TrimLeft::evaluate(const std::string &val,
     Transaction *transaction) {
+        std::string value(val);
     return *ltrim(&value);
 }
 
diff --git a/src/actions/transformations/trim_left.h b/src/actions/transformations/trim_left.h
index dedfd1d0..e7799208 100644
--- a/src/actions/transformations/trim_left.h
+++ b/src/actions/transformations/trim_left.h
@@ -32,7 +32,8 @@ namespace transformations {
 class TrimLeft : public Trim {
  public:
     explicit TrimLeft(const std::string &action) ;
-    std::string evaluate(std::string exp,
+
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 };
 
diff --git a/src/actions/transformations/trim_right.cc b/src/actions/transformations/trim_right.cc
index 1c77a1b7..bb7cac08 100644
--- a/src/actions/transformations/trim_right.cc
+++ b/src/actions/transformations/trim_right.cc
@@ -36,8 +36,9 @@ TrimRight::TrimRight(const std::string &action)
     this->action_kind = 1;
 }
 
-std::string TrimRight::evaluate(std::string value,
+std::string TrimRight::evaluate(const std::string &val,
     Transaction *transaction) {
+    std::string value(val);
     return *this->rtrim(&value);
 }
 
diff --git a/src/actions/transformations/trim_right.h b/src/actions/transformations/trim_right.h
index dca5bef7..8c100928 100644
--- a/src/actions/transformations/trim_right.h
+++ b/src/actions/transformations/trim_right.h
@@ -32,7 +32,8 @@ namespace transformations {
 class TrimRight : public Trim {
  public:
     explicit TrimRight(const std::string &action) ;
-    std::string evaluate(std::string exp,
+
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 };
 
diff --git a/src/actions/transformations/upper_case.cc b/src/actions/transformations/upper_case.cc
index 91256e50..0ae77c0e 100644
--- a/src/actions/transformations/upper_case.cc
+++ b/src/actions/transformations/upper_case.cc
@@ -31,8 +31,9 @@ UpperCase::UpperCase(const std::string &a)
     : Transformation(a) {
 }
 
-std::string UpperCase::evaluate(std::string value,
+std::string UpperCase::evaluate(const std::string &val,
     Transaction *transaction) {
+    std::string value(val);
     std::locale loc;
 
     for (std::string::size_type i=0; i < value.length(); ++i) {
diff --git a/src/actions/transformations/upper_case.h b/src/actions/transformations/upper_case.h
index 70cf2e70..542df283 100644
--- a/src/actions/transformations/upper_case.h
+++ b/src/actions/transformations/upper_case.h
@@ -33,7 +33,8 @@ namespace transformations {
 class UpperCase : public Transformation {
  public:
     explicit UpperCase(const std::string &action) ;
-    std::string evaluate(std::string exp,
+
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 };
 
diff --git a/src/actions/transformations/url_decode.cc b/src/actions/transformations/url_decode.cc
index f721b784..c533b7e8 100644
--- a/src/actions/transformations/url_decode.cc
+++ b/src/actions/transformations/url_decode.cc
@@ -37,7 +37,7 @@ UrlDecode::UrlDecode(const std::string &action)
     this->action_kind = 1;
 }
 
-std::string UrlDecode::evaluate(std::string value,
+std::string UrlDecode::evaluate(const std::string &value,
     Transaction *transaction) {
     unsigned char *val = NULL;
     int invalid_count = 0;
diff --git a/src/actions/transformations/url_decode.h b/src/actions/transformations/url_decode.h
index 26a0e2a7..9e39045b 100644
--- a/src/actions/transformations/url_decode.h
+++ b/src/actions/transformations/url_decode.h
@@ -32,8 +32,10 @@ namespace transformations {
 
 class UrlDecode : public Transformation {
  public:
+
     explicit UrlDecode(const std::string &action) ;
-    std::string evaluate(std::string exp,
+
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 };
 
diff --git a/src/actions/transformations/url_decode_uni.cc b/src/actions/transformations/url_decode_uni.cc
index 0485a03c..95331250 100644
--- a/src/actions/transformations/url_decode_uni.cc
+++ b/src/actions/transformations/url_decode_uni.cc
@@ -38,7 +38,7 @@ namespace actions {
 namespace transformations {
 
 
-std::string UrlDecodeUni::evaluate(std::string value,
+std::string UrlDecodeUni::evaluate(const std::string &value,
     Transaction *t) {
     std::string ret;
     unsigned char *input;
diff --git a/src/actions/transformations/url_decode_uni.h b/src/actions/transformations/url_decode_uni.h
index 6bc7415b..b04aef89 100644
--- a/src/actions/transformations/url_decode_uni.h
+++ b/src/actions/transformations/url_decode_uni.h
@@ -33,7 +33,7 @@ class UrlDecodeUni : public Transformation {
  public:
     explicit UrlDecodeUni(const std::string &action)  : Transformation(action) { }
 
-    std::string evaluate(std::string exp, Transaction *transaction) override;
+    std::string evaluate(const std::string &exp, Transaction *transaction) override;
     static int inplace(unsigned char *input, uint64_t input_len,
         Transaction *transaction);
 };
diff --git a/src/actions/transformations/url_encode.cc b/src/actions/transformations/url_encode.cc
index a83028b4..23a8f167 100644
--- a/src/actions/transformations/url_encode.cc
+++ b/src/actions/transformations/url_encode.cc
@@ -87,7 +87,7 @@ std::string UrlEncode::url_enc(const char *input,
 }
 
 
-std::string UrlEncode::evaluate(std::string value,
+std::string UrlEncode::evaluate(const std::string &value,
     Transaction *transaction) {
     int changed;
 
diff --git a/src/actions/transformations/url_encode.h b/src/actions/transformations/url_encode.h
index 3f56ec48..438301f7 100644
--- a/src/actions/transformations/url_encode.h
+++ b/src/actions/transformations/url_encode.h
@@ -30,8 +30,10 @@ namespace transformations {
 
 class UrlEncode : public Transformation {
  public:
+
     explicit UrlEncode(const std::string &action) ;
-    std::string evaluate(std::string exp,
+
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 
     std::string url_enc(const char *input,
diff --git a/src/actions/transformations/utf8_to_unicode.cc b/src/actions/transformations/utf8_to_unicode.cc
index d9f4f4e0..38d9a2b7 100644
--- a/src/actions/transformations/utf8_to_unicode.cc
+++ b/src/actions/transformations/utf8_to_unicode.cc
@@ -33,7 +33,7 @@ namespace actions {
 namespace transformations {
 
 
-std::string Utf8ToUnicode::evaluate(std::string value,
+std::string Utf8ToUnicode::evaluate(const std::string &value,
     Transaction *transaction) {
     std::string ret;
     unsigned char *input;
diff --git a/src/actions/transformations/utf8_to_unicode.h b/src/actions/transformations/utf8_to_unicode.h
index 038f8a14..c76bb0a2 100644
--- a/src/actions/transformations/utf8_to_unicode.h
+++ b/src/actions/transformations/utf8_to_unicode.h
@@ -37,7 +37,7 @@ class Utf8ToUnicode : public Transformation {
  public:
     explicit Utf8ToUnicode(const std::string &action)  : Transformation(action) { }
 
-    std::string evaluate(std::string exp,
+    std::string evaluate(const std::string &exp,
         Transaction *transaction) override;
 
     static char *inplace(unsigned char *input, uint64_t input_len,
diff --git a/src/utils/md5.cc b/src/utils/md5.cc
index 0601bf93..84746007 100644
--- a/src/utils/md5.cc
+++ b/src/utils/md5.cc
@@ -7,7 +7,7 @@ namespace modsecurity {
 namespace Utils {
 
 
-std::string Md5::hexdigest(std::string& input) {
+std::string Md5::hexdigest(const std::string& input) {
     unsigned char digest[16];
 
     mbedtls_md5(reinterpret_cast<const unsigned char *>(input.c_str()),
@@ -22,7 +22,7 @@ std::string Md5::hexdigest(std::string& input) {
 }
 
 
-std::string Md5::digest(std::string& input) {
+std::string Md5::digest(const std::string& input) {
     unsigned char output[16];
     std::string ret;
 
diff --git a/src/utils/md5.h b/src/utils/md5.h
index a7cfd85b..6720a8b3 100644
--- a/src/utils/md5.h
+++ b/src/utils/md5.h
@@ -31,8 +31,8 @@ class Md5 {
  public:
     Md5() { }
 
-    static std::string hexdigest(std::string& input);
-    static std::string digest(std::string& input);
+    static std::string hexdigest(const std::string& input);
+    static std::string digest(const std::string& input);
 };
 
 }  // namespace Utils