ValidNumber.cpp

bool SkipIfChar(const char* str, const char** newPos, char c)
{
    const char* pos = str;
    while(*pos == c)
        ++pos;
    *newPos = pos;
    return pos != str;
}

bool SkipIfNextChar(const char* str, const char** newPos, char c)
{
    if (*str == c)
    {
        *newPos = ++str;
        return true;
    }
    return false;
}

bool SkipIfSpace(const char* str, const char** newPos)
{
    return SkipIfChar(str, newPos, ' ');
}

bool SkipIfDigit(const char* str, const char** newPos)
{
    const char* pos = str;
    while(*pos <= '9' && *pos >= '0')
        ++pos;
    *newPos = pos;
    return pos != str;
}

bool SkipIfSign(const char* str, const char** newPos) 
{ 
     const char* pos = str;
     if(*pos == '+' || *pos == '-') 
        ++pos;
     *newPos = pos;
     return pos != str;
}

bool SkipIfExp(const char* str, const char** newPos)
{
    SkipIfSign(str, &str);
    if (SkipIfDigit(str, newPos))
        return true;
    return false;
}

bool SkipIfInteger(const char* str, const char** newPos)
{
    return SkipIfDigit(str, newPos) && (!SkipIfNextChar(*newPos, newPos, 'e') || SkipIfExp(*newPos, newPos));
}

bool IsEnd(const char* str)
{
    SkipIfSpace(str, &str);
    return *str == '\0';
}

bool isNumber(const char* s) 
{
    SkipIfSpace(s, &s);
    SkipIfSign(s, &s);
    if (SkipIfNextChar(s, &s, '.'))
    {
        return SkipIfInteger(s, &s) && IsEnd(s);
    }
    else if (SkipIfDigit(s, &s))
    {
        if (SkipIfNextChar(s, &s, '.'))
        {
            SkipIfDigit(s, &s);
            return (!SkipIfNextChar(s, &s, 'e') || SkipIfExp(s, &s)) && IsEnd(s);
        }
        else if (SkipIfNextChar(s, &s, 'e'))
            return SkipIfExp(s, &s) && IsEnd(s);
        return IsEnd(s);
    }
    return false;
}