C ++正则表达式子字符串发现错误模式

Question

I'm trying to understand the logic on the regex in c++

std::string s ("Ni Ni Ni NI");
std::regex e ("(Ni)");

std::smatch sm;  
std::regex_search (s,sm,e);
std::cout << "string object with " << sm.size() << " matches\n"; 

This form shouldn't give me the number of substrings matching my pattern? Because it always give me 1 match and it says that the match is [Ni , Ni]; but i need it to find every single pattern; they should be 3 and like this [Ni][Ni][Ni]

Answer 1

The function std::regex_search only returns the results for the first match found in your string.

Here is a code, merged from yours and from cplusplus.com . The idea is to search for the first match, analyze it, and then start again using the rest of the string (that is to say, the sub-string that directly follows the match that was found, which can be retrieved thanks to match_results::suffix ).

Note that the regex has two capturing groups (Ni*) and ([^ ]*) .

std::string s("the knights who say Niaaa and Niooo");
std::smatch m;
std::regex e("(Ni*)([^ ]*)");

while (std::regex_search(s, m, e))
{
    for (auto x : m)
        std::cout << x.str() << " ";

    std::cout << std::endl;
    s = m.suffix().str();
}

This gives the following output:

Niaaa Ni aaa

Niooo Ni ooo

As you can see, for every call to regex_search, we have the following information:

• the content of the whole match,
• the content of every capturing group.

Since we have two capturing groups, this gives us 3 strings for every regex_search.

EDIT : in your case if you want to retrieve every "Ni", all you need to do is to replace

std::regex e("(Ni*)([^ ]*)");

with

std::regex e("(Ni)");

You still need to iterate over your string, though.