fix(stdlib): reject NaN, infinity and hexadecimal text in IS_NUM, STR_TO_NUM and INPUT

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

2 files changed, 6 insertions, 2 deletions

diff --git a/pseudocode-lib/README.md b/pseudocode-lib/README.md
index dbdebf3..a12620a 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 CHAR containing the low eight bits. `LENGTH(CHR(0))` is 1.
 - `NUM_TO_STR(INTEGER)` and `NUM_TO_STR(REAL)` convert INTEGER to decimal text and REAL to the shortest round-trip text (Python `repr` style, identical to OUTPUT: `2.0`, `0.1`, `1e-07`, `inf`).
-- `STR_TO_NUM(S)` accepts STRING or CHAR and 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.
+- `STR_TO_NUM(S)` accepts STRING or CHAR and returns REAL through the same `parseNumber` helper as IS_NUM (`strtod`, full consumption, decimal text only: nan, inf/infinity and hexadecimal are rejected). 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; nan, inf/infinity, hexadecimal, 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 4395304..a20bebe 100644
--- a/pseudocode-lib/string.cc
+++ b/pseudocode-lib/string.cc
@@ -49,6 +49,10 @@ 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;
+    // Only decimal text: reject nan, inf/infinity and hexadecimal forms that strtod accepts.
+    if (Source.find_first_not_of("0123456789.eE+-", Source.find_first_not_of(" \t\n\v\f\r")) !=
+        std::string::npos)
+        return NumberStatus::Invalid;
     char *End = nullptr;
     errno = 0;
     double Number = std::strtod(Source.c_str(), &End);