regex_replace is returning empty string - c++

Question

I am trying to remove all characters that are not digit, dot (.), plus/minus sign (+/-) with empty character/string for float conversion.
When I pass my string through regex_replace function I am returned an empty string.
I belive something is wrong with my regex expression std::regex reg_exp("\\D|[^+-.]")
Code
#include <iostream>
#include <regex>
int main()
{
std::string temporary_recieve_data = " S S +456.789 tg\r\n";
std::string::size_type sz;
const std::regex reg_exp("\\D|[^+-.]"); // matches not digit, decimal point (.), plus sign, minus sign
std::string numeric_string = std::regex_replace(temporary_recieve_data, reg_exp, ""); //replace the character that are not digit, dot (.), plus-minus sign (+,-) with empty character/string for float conversion
std::cout << "Numeric String : " << numeric_string << std::endl;
if (numeric_string.empty())
{
return 0;
}
float data_value = std::stof(numeric_string, &sz);
std::cout << "Float Value : " << data_value << std::endl;
return 0;
}
I have been trying to evaluate my regex expression on regex101.com for past 2 days but I am unable to figure out where I am wrong with my regular expression. When I just put \D, the editor substitutes non-digit character properly but soon as I add or condition | for not dot . or plus + or minus - sign the editor returns empty string.

Answer 1

The string is empty because your regex matches each character.
\D already matches every character that is not a digit.
So plus, hyphen and the period thus far are consumed.
And digits get consumed by the negated class: [^+-.]
Further the hyphen indicates a range inside a character class.
Either escape it or put it at the start or end of the char-class.
(funnily the used range +-. 43-46 even contained a hyphen)
Remove the alternation with \D and put \d into the negated class:
[^\d.+-]+
See this demo at regex101 (attaching + for one or more is efficient)