正則表達式不返回任何結果

Question

我有幾個關於boost :: regex的問題：我嘗試了下面的一個例子。

1）什麼是sregex_token_iterator的第4個參數？它聽起來像一個「默認匹配」，但你爲什麼要這樣做，而不是什麼都不返回？我嘗試沒有第四個參數，但它不編譯。

2）我得到的輸出： （1,0） （0，0） （3,0） （0，0） （5,0）

誰能解釋一下會出錯？

#include <iostream> 
#include <sstream> 
#include <vector> 
#include <boost/regex.hpp> 

// This example extracts X and Y from (X , Y), (X,Y), (X, Y), etc. 


struct Point 
{ 
    int X; 
    int Y; 
    Point(int x, int y): X(x), Y(y){} 
}; 

typedef std::vector<Point> Polygon; 

int main() 
{ 
    Polygon poly; 
    std::string s = "Polygon: (1.1,2.2), (3, 4), (5,6)"; 

    std::string floatRegEx = "[0-9]*\\.?[0-9]*"; // zero or more numerical characters as you want, then an optional '.', then zero or more numerical characters. 
    // The \\. is for \. because the first \ is the c++ escpape character and the second \ is the regex escape character 
    //const boost::regex r("(\\d+),(\\d+)"); 
    const boost::regex r("(\\s*" + floatRegEx + "\\s*,\\s*" + floatRegEx + "\\s*)"); 
    // \s is white space. We want this to allow (2,3) as well as (2, 3) or (2 , 3) etc. 

    const boost::sregex_token_iterator end; 
    std::vector<int> v; // This type has nothing to do with the type of objects you will be extracting 
    v.push_back(1); 
    v.push_back(2); 

    for (boost::sregex_token_iterator i(s.begin(), s.end(), r, v); i != end;) 
    { 
    std::stringstream ssX; 
    ssX << (*i).str(); 
    float x; 
    ssX >> x; 
    ++i; 

    std::stringstream ssY; 
    ssY << (*i).str(); 
    float y; 
    ssY >> y; 
    ++i; 

    poly.push_back(Point(x, y)); 
    } 

    for(size_t i = 0; i < poly.size(); ++i) 
    { 
    std::cout << "(" << poly[i].X << ", " << poly[i].Y << ")" << std::endl; 
    } 
    std::cout << std::endl; 

    return 0; 
} 

Answer 1

你的正則表達式是完全可選：

"[0-9]*\\.?[0-9]*" 

也匹配空字符串。所以"(\\s*" + floatRegEx + "\\s*,\\s*" + floatRegEx + "\\s*)"也匹配一個逗號。

你應該至少強制的事情：

"(?:[0-9]+(?:\\.[0-9]*)?|\\.[0-9]+)" 

這使得1，1.1，1.或.1但不.：

(?:   # Either match... 
[0-9]+  # one or more digits, then 
(?:   # try to match... 
    \.   # a dot 
    [0-9]*  # and optional digits 
)?   # optionally. 
|   # Or match... 
\.[0-9]+ # a dot and one or more digits. 
)   # End of alternation