2 files changed, 28 insertions, 11 deletions
diff --git a/pseudocode-lib/README.md b/pseudocode-lib/README.md
index e0819bf..1e360a4 100644
--- a/pseudocode-lib/README.md
+++ b/pseudocode-lib/README.md
@@ -35,7 +35,7 @@ The library provides `&`, six case-sensitive content comparisons, and these ordi
 - `ASC(CHAR)` and `ASC(STRING)` return the unsigned character or first-byte code; an empty string returns zero.
 - `CHR(Code)` returns a one-byte STRING containing the low eight bits. `LENGTH(CHR(0))` is 1.
 - `NUM_TO_STR(INTEGER)` and `NUM_TO_STR(REAL)` use C++17 `std::to_string`, with six fractional digits for REAL.
-- `STR_TO_NUM(S)` returns REAL using `std::stod` and requires full consumption. Invalid text, trailing junk, and out-of-range values throw.
-- `IS_NUM(S)` tests full consumption by `strtod` without throwing for invalid text. Leading whitespace is accepted; trailing whitespace, trailing junk, empty input, and embedded NUL bytes are rejected.
+- `STR_TO_NUM(S)` returns REAL through the same `parseNumber` helper as IS_NUM (`strtod`, full consumption). Invalid text and trailing junk throw `STR_TO_NUM: '<text>' is not a number`; overflow throws `... is out of the REAL range`; underflow is accepted.
+- `IS_NUM(S)` agrees with STR_TO_NUM: full consumption by `strtod` and no overflow, without throwing. Leading whitespace is accepted; trailing whitespace, trailing junk, empty input, embedded NUL bytes, and overflowing values (`1e999`) are rejected.
 
 Library C++ exceptions print the user call line and can be caught with TRY/CATCH. Library functions follow the guide where specified; LEFT, STRING case conversion, byte-code and numeric-conversion functions remain compatibility extensions.
diff --git a/pseudocode-lib/string.cc b/pseudocode-lib/string.cc
index f2f03f6..34f1ec5 100644
--- a/pseudocode-lib/string.cc
+++ b/pseudocode-lib/string.cc
@@ -1,6 +1,8 @@
 #include <algorithm>
 #include <cctype>
 #include <cstdio>
+#include <cerrno>
+#include <cmath>
 #include <cstdlib>
 #include <exception>
 #include <stdexcept>
@@ -20,6 +22,20 @@ decltype(auto) checked(Operation &&Op) {
     }
 }
 std::string &text(void *Handle) { return *static_cast<std::string *>(Handle); }
+
+enum class NumberStatus { Ok, Invalid, OutOfRange };
+
+// Shared by IS_NUM and STR_TO_NUM so that they always agree.
+NumberStatus parseNumber(const std::string &Source, double &Result) {
+    if (Source.empty()) return NumberStatus::Invalid;
+    char *End = nullptr;
+    errno = 0;
+    double Number = std::strtod(Source.c_str(), &End);
+    if (End != Source.c_str() + Source.size()) return NumberStatus::Invalid;
+    if (errno == ERANGE && std::isinf(Number)) return NumberStatus::OutOfRange;
+    Result = Number;
+    return NumberStatus::Ok;
+}
 }
 
 extern "C" {
@@ -58,11 +74,14 @@ void _string_impl_from_real(void *Target, double Number) {
 }
 double _string_impl_to_real(void *Handle) {
     return checked([&] {
-        std::size_t Consumed = 0;
-        double Number = std::stod(text(Handle), &Consumed);
-        if (Consumed != text(Handle).size())
-            throw std::invalid_argument("unconsumed characters in string conversion");
-        return Number;
+        double Number = 0;
+        switch (parseNumber(text(Handle), Number)) {
+        case NumberStatus::Ok: return Number;
+        case NumberStatus::OutOfRange:
+            throw std::out_of_range("STR_TO_NUM: '" + text(Handle) + "' is out of the REAL range");
+        case NumberStatus::Invalid: break;
+        }
+        throw std::invalid_argument("STR_TO_NUM: '" + text(Handle) + "' is not a number");
     });
 }
 long long _string_impl_length(void *Handle) {
@@ -130,10 +149,8 @@ void _string_impl_chr(void *Target, long long Code) {
 }
 int _string_impl_is_num(void *Handle) {
     return checked([&]() -> int {
-        const auto &Source = text(Handle);
-        char *End = nullptr;
-        std::strtod(Source.c_str(), &End);
-        return !Source.empty() && End == Source.c_str() + Source.size();
+        double Number = 0;
+        return parseNumber(text(Handle), Number) == NumberStatus::Ok;
     });
 }
 const char *_string_impl_c_str(void *Handle) {