Validating integer part of an string - c++

Question

I have a text file that I need to convert each line into integer.
The lines can begin with '#' to indicate a comment. Also, after the data it might be inline comment too...again indicated by '#'
So I have the example below:
QString time = "5000 #this is 5 seconds"; // OK
QString time = " 5000 # this is 5 seconds"; // OK..free spaceis allowed at start
QString time = "5000.00 #this is 5 seconds"; // invalid...no decimal
QString time = "s5000 # this is 5 seconds"; // invalid...does not start with numerical character
How can I take care of these cases? I mean in all the 4 example above except the last two I need to extract "5000". How to find out the last one is invalid?
So I mean what is the best fail-proof code to handle this task?

Answer 1

Another example using std::regex. Converting QString to a string_view is left as an exercise for the reader.
#include <regex>
#include <string_view>
#include <iostream>
#include <string>
#include <optional>
std::optional<std::string> extract_number(std::string_view input)
{
static constexpr char expression[] = R"xx(^\s*(\d+)\s*(#.*)?$)xx";
static const auto re = std::regex(expression);
auto result = std::optional<std::string>();
auto match = std::cmatch();
const auto matched = std::regex_match(input.begin(), input.end(), match, re);
if (matched)
{
result.emplace(match[1].first, match[1].second);
}
return result;
}
void emit(std::string_view candidate, std::optional<std::string> result)
{
std::cout << "offered: " << candidate << " - result : " << result.value_or("no match") << '\n';
}
int main()
{
const std::string_view candidates[] =
{
"5000 #this is 5 seconds",
" 5000 # this is 5 seconds",
"5000.00 #this is 5 seconds",
"s5000 # this is 5 seconds"
};
for(auto candidate : candidates)
{
emit(candidate, extract_number(candidate));
}
}
expected output:
offered: 5000 #this is 5 seconds - result : 5000
offered: 5000 # this is 5 seconds - result : 5000
offered: 5000.00 #this is 5 seconds - result : no match
offered: s5000 # this is 5 seconds - result : no match
https://coliru.stacked-crooked.com/a/2b0e088e6ed0576b

Answer 2

You can use this regex to validate and extract the digit from first grouping pattern that will capture your number,
^\s*(\d+)\b(?!\.)
Explanation:
^ - Start of string
\s* - Allows optional space before the number
(\d+) - Captures the number and places it in first grouping pattern
\b - Ensures the number does not match partially in a larger text because of the negative look ahead present ahead
(?!\.) - Rejects the match if there is a decimal following the number
Demo1
In case only last one is invalid, you can use this regex to capture number from first three entries,
^\s*(\d+)
Demo2