std :: regex_match& std :: regex_search？ [英] Difference between std::regex_match & std::regex_search?

问题描述

以下程序已写入，以使用C ++ 11获取日信息 std :: regex_match & std :: regex_search 。但是，使用第一个方法返回 false ，第二个方法返回 true （预期）。我阅读文档和已经存在的SO相关的问题，但我不明白这两种方法之间的区别，当我们应该使用它们中的任何一个？他们可以互换使用任何常见的问题吗？

Below program has been written to fetch the "Day" information using the C++11 std::regex_match & std::regex_search. However, using the first method returns false and second method returns true(expected). I read the documentation and already existing SO question related to this, but I do not understand the difference between these two methods and when we should use either of them? Can they both be used interchangeably for any common problem?

regex_match和regex_search？

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

int main()
{
    std::string input{ "Mon Nov 25 20:54:36 2013" };
    //Day:: Exactly Two Number surrounded by spaces in both side
    std::regex  r{R"(\s\d{2}\s)"};
    //std::regex  r{"\\s\\d{2}\\s"};
    std::smatch match;

if (std::regex_match(input,match,r)) {
        std::cout << "Found" << "\n";
    } else {
        std::cout << "Did Not Found" << "\n";
    }

    if (std::regex_search(input, match,r)) {
        std::cout << "Found" << "\n";
        if (match.ready()){
            std::string out = match[0];
            std::cout << out << "\n";
        }
    }
    else {
        std::cout << "Did Not Found" << "\n";
    }
}

输出

Output

Did Not Found

Found

 25 

为什么在这种情况下，第一个regex方法返回 false ？ regex 似乎是正确的，因此理想情况下两个都应该返回 true 。我通过改变 std :: regex_match（input，match，r）到 std :: regex_match（input，r） code>并发现它仍然返回 false。

Why first regex method returns false in this case?. The regex seems to be correct so ideally both should have been returned true. I ran the above program by changing the std::regex_match(input,match,r) to std::regex_match(input,r) and found that it still returns false.

有人可以解释上面的例子，一般，这些方法的用例？

Could somebody explain the above example and, in general, use cases of these methods?

推荐答案

regex_match code> true ，而 regex_search 会成功，即使只有一个子序列匹配 regex 。

regex_match only returns true when the entire input sequence has been matched, while regex_search will succeed even if only a sub-sequence matches the regex.

引用N3337，

§28.11.2/ 2 regex_match [re.alg.match]


/ em>确定正则表达式 e 和所有字符序列之间是否匹配[first，last] code> 。 ... 如果存在此类匹配，则返回 true false

§28.11.2/2 regex_match [re.alg.match]

Effects: Determines whether there is a match between the regular expression e, and all of the character sequence [first,last). ... Returns true if such a match exists, false otherwise.

上面的描述是针对 regex_match 到要匹配的序列的迭代器对。

The above description is for the regex_match overload that takes a pair of iterators to the sequence to be matched. The remaining overloads are defined in terms of this overload.

相应的 regex_search 重载描述为

§28.11.3/ 2 regex_search .alg.search]



效果：确定中是否有 ，last） 匹配正则表达式 e 。 ... 返回 true 如果存在这样的序列， false

§28.11.3/2 regex_search [re.alg.search]

Effects: Determines whether there is some sub-sequence within [first,last) that matches the regular expression e. ... Returns true if such a sequence exists, false otherwise.

---

在您的示例中，如果修改 regex 到 r {R（。*？\s\d {2} \s。*）}; regex_match 和 regex_search 将成功（但匹配结果不仅仅是日期，而是整个日期字符串）。

---

In your example, if you modify the regex to r{R"(.*?\s\d{2}\s.*)"}; both regex_match and regex_search will succeed (but the match result is not just the day, but the entire date string).

现场演示您的示例的修改版本该日被 regex_match 和 regex_search 捕获和显示。

Live demo of a modified version of your example where the day is being captured and displayed by both regex_match and regex_search.

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