Fast way to determine if string is a JSON number - c++

Question

The check must not user C++11 features or additional libraries e.g. Boost, Regex etc. I came up with the following solution. No pretty but working. Does it get more elegant and/or faster from here?
bool isJsonNumber(const std::string& text)
{
if(text.empty()) return false;
bool foundE = false;
bool foundESign = false;
bool leadingZero = false;
bool lastIsDigit = false;
bool foundDot = false;
for(uint32_t i=0; i < text.length(); ++i)
{
const unsigned char c = text[i];
lastIsDigit = false;
const bool currIsNoDigit = (c < '0' || c > '9');
if(i == 0)
{
if(currIsNoDigit && c != '-' ) return false;
if(c == '0') leadingZero = true;
if(c != '-') lastIsDigit = true;
}
else
{
if(leadingZero)
{
leadingZero = false;
if(c != '.') return false;
foundDot = true;
}
else if(c == '.')
{
if(foundDot) return false;
foundDot = true;
}
else if(c == 'e' || c == 'E')
{
if(foundE) return false;
foundE = true;
}
else if(foundE && !foundESign)
{
if(currIsNoDigit && c != '-' && c != '+') return false;
if(c == '+' || c == '-')
{
foundESign = true;
}
else
{
lastIsDigit = true;
}
}
else
{
foundESign = false;
if(currIsNoDigit) return false;
lastIsDigit = true;
}
}
}
if(lastIsDigit == false) return false;
return true;
}
The use case is a small embedded server that recieves huge CSV files and anwsers clients with portions in JSON.

Answer 1

It would probably be easier to use std::stod:
size_t endpos;
std::stod(text, &endpos);
if (endpos != text.length())
{
// Not a number
}
else
{
// A number
}
If you don't have std::stod, as it's a C++11 feature, you could do something similar with std::strtod.
If you want to disallow INFINITY or NAN or hexadecimal floating point values, it's as easy as checking that the second or third character in the string is not a letter:
if ((text.length() > 2 && std::isalpha(text[1])) ||
(text.length() > 3 && std::isalpha(text[2])))
{
// Not a number
}
For "larger" numnbers there always std::stold or std::strtold. However if you want arbitrarily sized numbers then either do as you do now or use a library such as GMP (mpf_set_str seems like a good function for this).