C++ 通过一个简单的例子理解c++正则表达式
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Understanding c++ regex by a simple example
提问by
I wrote the following simple example:
我写了以下简单的例子：
#include <iostream>
#include <string>
#include <regex>
int main ()
{
std::string str("1231");
std::regex r("^(\d)");
std::smatch m;
std::regex_search(str, m, r);
for(auto v: m) std::cout << v << std::endl;
}
and got confused by its behavior. If I understood the purpose of the match_resultfrom therecorrectly, the only one 1should have been printed. Actually:
并对其行为感到困惑。如果我match_result从那里正确理解了 的目的，那么1应该打印唯一一个。实际上：
If successful, it is not empty and contains a series of sub_match objects: the first sub_match element corresponds to the entire match,and, if the regex expression contained sub-expressions to be matched ([...])
如果成功，则它不为空并包含一系列 sub_match 对象：第一个 sub_match 元素对应整个匹配，并且，如果正则表达式包含要匹配的子表达式 ([...])
The string passed to the function doesn't match the regex, therefore we should nothave had the entire match.
传递给函数的字符串与正则表达式不匹配，因此我们不应该有the entire match.
What did I miss?
我错过了什么？
采纳答案by Galik
You still get the entire matchbut the entire matchdoes not fit the entire stringit fits the entire regex.
你仍然得到整个匹配，但整个匹配不适合整个字符串它适合整个 regex。
For example consider this:
例如考虑这个：
#include <iostream>
#include <string>
#include <regex>
int main()
{
std::string str("1231");
std::regex r("^(\d)\d"); // entire match will be 2 numbers
std::smatch m;
std::regex_search(str, m, r);
for(auto v: m)
std::cout << v << std::endl;
}
Output:
输出：
12
1
The entire match(first sub_match) is what the entire regexmatches against (part of the string).
在整场比赛（第一sub_match）是什么，整个正则表达式对（字符串的一部分）相匹配。
The second sub_match is the first (and only) capture group
第二个 sub_match 是第一个（也是唯一一个）捕获组
Looking at your original regex
查看您的原始正则表达式
std::regex r("^(\d)");
|----| <- entire expression (sub_match #0)
std::regex r("^(\d)");
|---| <- first capture group (sub_match #1)
That is where the two sub_matchescome from.
这就是两个sub_matches 的来源。
回答by shockawave123
From here
从这里
Returns whether **some** sub-sequence in the target sequence (the subject)
matches the regular expression rgx (the pattern). The target sequence is
either s or the character sequence between first and last, depending on
the version used.
So regex_search will search for anything in the input string that matches the regex. The whole string doesnt have to match, just part of it.
因此 regex_search 将搜索输入字符串中与正则表达式匹配的任何内容。整个字符串不必匹配，只需匹配其中的一部分。
However, if you were to use regex_match, then the entirestring must match.
但是，如果您要使用 regex_match，则整个字符串必须匹配。
