Removing need for RegEx when expanding ENV variables

src/cpp/xmlparser/XMLElementParser.cpp

@@ -48,65 +48,82 @@ static std::string process_environment(
      * Environment variable names used ... consist solely of uppercase letters, digits, and the '_' (underscore)
      * from the characters defined in Portable Character Set and do not begin with a digit.
      */
-    std::regex expression("\\$\\{([A-Z_][A-Z0-9_]*)\\}");
-
-    // Algorithm inspired by https://stackoverflow.com/a/37516316/559350
+    enum class ParseStage : uint8_t {
+        PLAIN_CHAR,
+        DOLLAR,
+        OPEN_CURLY,
+        ENV_CHAR,
+    };
 
     // This will hold the accumulated substitution result
     std::string ret_val{};
 
-    // Iterators to first and last character of the input string
-    auto first = input.cbegin();
-    auto last = input.cend();
-
-    // Position of last match in the input string
-    std::smatch::difference_type last_match_position = 0;
+    bool all_expanded = true;
+    const char* cursor = input.c_str();
 
-    // Iterator to the character after the last match in the input string
-    auto last_match_end = first;
+    ParseStage parse_stage = ParseStage::PLAIN_CHAR;
+    const char* match_start = cursor;
+    const char* last_match_end = cursor;
+    std::string var_name;
 
-    // Functor called to process each match
-    auto match_cb = [&](const std::smatch& match)
-            {
-                // Compute substitution value
-                std::string var_name = match[1];
-                std::string value = "";
-                if (var_name == "_")
-                {
-                    // Silently ignore ${_} since it might expose sensitive information (full path to executable).
-                    EPROSIMA_LOG_WARNING(XMLPARSER, "Ignoring environment variable ${_}");
+    for (; *cursor != '\0'; ++cursor) {
+        switch (parse_stage) {
+            case ParseStage::PLAIN_CHAR:
+                if ('$' == *cursor) {
+                    parse_stage = ParseStage::DOLLAR;
                 }
-                else if (dds::RETCODE_OK != SystemInfo::get_env(var_name, value))
-                {
-                    EPROSIMA_LOG_ERROR(XMLPARSER, "Could not find a value for environment variable " << var_name);
+                break;
+            case ParseStage::DOLLAR:
+                if ('{' == *cursor) {
+                    match_start = cursor + 1;
+                    parse_stage = ParseStage::OPEN_CURLY;
+                } else {
+                    parse_stage = ParseStage::PLAIN_CHAR;
+                }
+                break;
+            case ParseStage::OPEN_CURLY:
+                if ((0 != std::isupper(*cursor)) || ('_' == *cursor)) {
+                    // Must start with A-Z or '_'
+                    parse_stage = ParseStage::ENV_CHAR;
+                } else {
+                    parse_stage = ParseStage::PLAIN_CHAR;
                 }
+                break;
+            case ParseStage::ENV_CHAR:
+                if ('}' == *cursor) {
+                    // Append everything after last '}' (or beginning of input) up until '$'
+                    ret_val.append(last_match_end, match_start - 2);
 
-                // Compute number of non-matching characters between this match and the last one
-                auto this_match_position = match.position();
-                auto diff = this_match_position - last_match_position;
+                    // Extract the part between curly braces to simplify lookup
+                    var_name.assign(match_start, cursor);
 
-                // Append non-matching characters to return value
-                auto this_match_start = last_match_end;
-                std::advance(this_match_start, diff);
-                ret_val.append(last_match_end, this_match_start);
+                    std::string value = "";
 
-                // Append substitution value to return value
-                ret_val.append(value);
+                    if (var_name == "_")
+                    {
+                        // Silently ignore ${_} since it might expose sensitive information (full path to executable).
+                        EPROSIMA_LOG_WARNING(XMLPARSER, "Ignoring environment variable ${_}");
+                    }
+                    else if (dds::RETCODE_OK != SystemInfo::get_env(var_name, value))
+                    {
+                        EPROSIMA_LOG_ERROR(XMLPARSER, "Could not find a value for environment variable " << var_name);
+                    }
 
-                // Prepare for next iteration
-                auto match_length = match.length();
-                last_match_position = this_match_position + match_length;
-                last_match_end = this_match_start;
-                std::advance(last_match_end, match_length);
-            };
+                    ret_val.append(value);
 
-    // Substitute all matches
-    std::sregex_iterator begin(first, last, expression);
-    std::sregex_iterator end;
-    std::for_each(begin, end, match_cb);
+                    last_match_end = cursor + 1;
+                    parse_stage = ParseStage::PLAIN_CHAR;
+                } else if ((0 == std::isupper(*cursor)) && (0 == std::isdigit(*cursor)) &&
+                           ('_' != *cursor)) {
+                    // Can only be 0-9, A-Z, or '_'
+                    parse_stage = ParseStage::PLAIN_CHAR;
+                }
+                break;
+        }
+    }
 
-    // Append characters after last match
-    ret_val.append(last_match_end, last);
+    // Append any remaining characters (cursor at '\0')
+    ret_val.append(last_match_end, cursor);
 
     return ret_val;
 }