C ++正则表达式匹配'+'量词

Question

I want to match expression of the pattern

a space followed by a (addition operator or subtraction operator)

For example: " +" should return True

I have tried the using std::regex_match on the following regular exp:

" [+-]" , "\\\\s[+-]" , "\\\\s[+\\\\-]" , "\\\\s[\\\\+-]"

but they all return false.

What should be the correct expression ?

EDIT

Here's the test code:

#include<iostream>
#include<string>
#include<regex>

using std::cout;
int main()
{
    std::string input;
    std::cin>>input;
    const std::regex ex(" [\\+-]");
    std::smatch m;
    if( std::regex_match(input,ex))
    {
        cout<<"\nTrue";
    }
    else
        cout<<"\nFalse";
    return 0;
}

Answer 1

Now that you've posted code, we can see what the problem is:

std::string input;
std::cin >> input;

The problem is here, operator >> skips whitespace when it reads, so if you type space plus, cin will skip the space and then your regex will no longer match.

To get this program to work, use std::getline instead to read everything that was input before the user pressed enter (including spaces):

std::string input;
std::getline(std::cin, input);

Answer 2

尝试此操作，它将匹配带空格的字符串，后跟一个+或-。

" (\\+|-)"

Answer 3

Usually the minus sign is required to go first: [-abc] . This is because not to mix with ranges [ab] .

Answer 4

您可以尝试一下，因为我认为它应该可以工作，但我尚未对其进行测试： " [\\+-]"