inline bool isNumeric()

in include/llvm/Support/YAMLTraits.h [537:631]
64 lines of code
35 McCabe index (conditional complexity)

inline bool isNumeric(StringRef S) {
  const static auto skipDigits = [](StringRef Input) {
    return Input.drop_front(
        std::min(Input.find_first_not_of("0123456789"), Input.size()));
  };

  // Make S.front() and S.drop_front().front() (if S.front() is [+-]) calls
  // safe.
  if (S.empty() || S.equals("+") || S.equals("-"))
    return false;

  if (S.equals(".nan") || S.equals(".NaN") || S.equals(".NAN"))
    return true;

  // Infinity and decimal numbers can be prefixed with sign.
  StringRef Tail = (S.front() == '-' || S.front() == '+') ? S.drop_front() : S;

  // Check for infinity first, because checking for hex and oct numbers is more
  // expensive.
  if (Tail.equals(".inf") || Tail.equals(".Inf") || Tail.equals(".INF"))
    return true;

  // Section 10.3.2 Tag Resolution
  // YAML 1.2 Specification prohibits Base 8 and Base 16 numbers prefixed with
  // [-+], so S should be used instead of Tail.
  if (S.startswith("0o"))
    return S.size() > 2 &&
           S.drop_front(2).find_first_not_of("01234567") == StringRef::npos;

  if (S.startswith("0x"))
    return S.size() > 2 && S.drop_front(2).find_first_not_of(
                               "0123456789abcdefABCDEF") == StringRef::npos;

  // Parse float: [-+]? (\. [0-9]+ | [0-9]+ (\. [0-9]* )?) ([eE] [-+]? [0-9]+)?
  S = Tail;

  // Handle cases when the number starts with '.' and hence needs at least one
  // digit after dot (as opposed by number which has digits before the dot), but
  // doesn't have one.
  if (S.startswith(".") &&
      (S.equals(".") ||
       (S.size() > 1 && std::strchr("0123456789", S[1]) == nullptr)))
    return false;

  if (S.startswith("E") || S.startswith("e"))
    return false;

  enum ParseState {
    Default,
    FoundDot,
    FoundExponent,
  };
  ParseState State = Default;

  S = skipDigits(S);

  // Accept decimal integer.
  if (S.empty())
    return true;

  if (S.front() == '.') {
    State = FoundDot;
    S = S.drop_front();
  } else if (S.front() == 'e' || S.front() == 'E') {
    State = FoundExponent;
    S = S.drop_front();
  } else {
    return false;
  }

  if (State == FoundDot) {
    S = skipDigits(S);
    if (S.empty())
      return true;

    if (S.front() == 'e' || S.front() == 'E') {
      State = FoundExponent;
      S = S.drop_front();
    } else {
      return false;
    }
  }

  assert(State == FoundExponent && "Should have found exponent at this point.");
  if (S.empty())
    return false;

  if (S.front() == '+' || S.front() == '-') {
    S = S.drop_front();
    if (S.empty())
      return false;
  }

  return skipDigits(S).empty();
}