libexpr: avoid regex engine in getDerivations attr filtering

- getDerivations() filters attribute names with std::regex_match, which runs the regex engine for every attribute visited during nixpkgs scanning.
- BM_GetDerivationsAttrScan/10000_mean: 3.338 ms → 1.506 ms (≈ -54.9%)

src/libexpr/get-drvs.cc

@@ -367,7 +367,26 @@ static std::string addToPath(const std::string & s1, std::string_view s2)
     return s1.empty() ? std::string(s2) : s1 + "." + s2;
 }
 
-static std::regex attrRegex("[A-Za-z_][A-Za-z0-9-_+]*");
+static bool isAttrPathComponent(std::string_view symbol)
+{
+    if (symbol.empty())
+        return false;
+
+    /* [A-Za-z_] */
+    unsigned char first = symbol[0];
+    if (!((first >= 'A' && first <= 'Z') || (first >= 'a' && first <= 'z') || first == '_'))
+        return false;
+
+    /* [A-Za-z0-9-_+]* */
+    for (unsigned char c : symbol.substr(1)) {
+        if ((c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z') || (c >= '0' && c <= '9') || c == '-' || c == '_'
+            || c == '+')
+            continue;
+        return false;
+    }
+
+    return true;
+}
 
 static void getDerivations(
     EvalState & state,
@@ -400,7 +419,7 @@ static void getDerivations(
             std::string_view symbol{state.symbols[i->name]};
             try {
                 debug("evaluating attribute '%1%'", symbol);
-                if (!std::regex_match(symbol.begin(), symbol.end(), attrRegex))
+                if (!isAttrPathComponent(symbol))
                     continue;
                 std::string pathPrefix2 = addToPath(pathPrefix, symbol);
                 if (combineChannels)