C++ getline - 使用正則表達式提取子字符串

Question

我有一個包含這樣內容的文件 -

Random text
+-------------------+------+-------+-----------+-------+
|     Data          |   A  |   B   |     C     |   D   |
+-------------------+------+-------+-----------+-------+
|   Data 1          | 1403 |     0 |      2520 | 55.67 |
|   Data 2          | 1365 |     2 |      2520 | 54.17 |
|   Data 3          |    1 |     3 |      1234 | 43.12 |
Some more random text

我想提取行Data 1 D列的值，即我想從上面的示例中提取值55.67 。 我正在使用getline逐行解析此文件 -

while(getline(inputFile1,line)) {
    if(line.find("|  Data 1") != string::npos) {
        subString = //extract the desired value
}

如何從行中提取所需的子字符串。 有沒有辦法使用 boost::regex 來提取這個子字符串？

Answer 1

雖然regex可能有它的用途，但它可能對此有點過分。

引入trim功能和：

char delim;
std::string line, data;
int a, b, c;
double d;

while(std::getline(inputFile1, line)) {
    std::istringstream is(line);
    if( std::getline(is >> delim, data, '|') >>
        a >> delim >> b >> delim >> c >> delim >> d >> delim) 
    {
        trim(data);

        if(data == "Data 1") {
            std::cout << a << ' ' << b << ' ' << c << ' ' << d << '\n';
        }
    }
}

演示

Answer 2

是的，可以很容易地使用正則表達式提取您的子字符串。 無需使用boost，您也可以使用現有的C++ regex 庫。

由此產生的程序非常簡單。

我們在一個簡單的 for 循環中讀取源文件的所有行。 然后我們使用std::regex_match將剛剛讀取的行與我們的正則表達式相匹配。 如果我們找到了匹配項，那么結果將在std::smatch sm，組 1 中。

並且因為我們將設計用於查找雙精度值的正則表達式，所以我們將得到我們所需要的，沒有任何額外的空格。

我們可以將其轉換為 double 並在屏幕上顯示結果。 並且因為我們定義了正則表達式來查找雙std::stod ，所以我們可以確定std::stod會起作用。

生成的程序相當簡單易懂：

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

// Please note. For std::getline, it does not matter, if we read from a
// std::istringstream or a std::ifstream. Both are std::istream's. And because
// we do not have files here on SO, we will use an istringstream as data source.
// If you want to read from a file later, simply create an std::ifstream inputFile1

// Source File with all data
std::istringstream inputFile1{ R"(
Random text
+-------------------+------+-------+-----------+-------+
|     Data          |   A  |   B   |     C     |   D   |
+-------------------+------+-------+-----------+-------+
|   Data 1          | 1403 |     0 |      2520 | 55.67 |
|   Data 2          | 1365 |     2 |      2520 | 54.17 |
|   Data 3          |    1 |     3 |      1234 | 43.12 |
Some more random text)" 
};

// Regex for finding the desired data
const std::regex re(R"(\|\s+Data 1\s+\|.*?\|.*?\|.*?\|\s*([-+]?[0-9]*\.?[0-9]+)\s*\|)");

int main() {

    // The result will be in here
    std::smatch sm;

    // Read all lines of the source file
    for (std::string line{}; std::getline(inputFile1, line);) {

        // If we found our matching string
        if (std::regex_match(line, sm, re)) {

            // Then extract the column D info
            double data1D = std::stod(sm[1]);

            // And show it to the user.
            std::cout << data1D << "\n";
        }
    }
}

對於大多數人來說，棘手的部分是如何定義正則表達式。 有像Online regex tester 和 debugger這樣的頁面。 還有正則表達式的細分和可以理解的解釋。

對於我們的正則表達式

\|\s+Data 1\s+\|.*?\|.*?\|.*?\|\s*([-+]?[0-9]*\.?[0-9]+)\s*\|

我們得到以下解釋：

\|  
    matches the character | literally (case sensitive)
\s+
    matches any whitespace character (equal to [\r\n\t\f\v ])
    + Quantifier — Matches between one and unlimited times, as many times as possible, giving back as needed (greedy)
    Data 1 matches the characters Data 1 literally (case sensitive)
\s+
    matches any whitespace character (equal to [\r\n\t\f\v ])
    + Quantifier — Matches between one and unlimited times, as many times as possible, giving back as needed (greedy)
\| 
    matches the character | literally (case sensitive)
.*?
    matches any character (except for line terminators)
    *? Quantifier — Matches between zero and unlimited times, as few times as possible, expanding as needed (lazy)
\| 
    matches the character | literally (case sensitive)
.*?
    matches any character (except for line terminators)
    *? Quantifier — Matches between zero and unlimited times, as few times as possible, expanding as needed (lazy)
\| 
    matches the character | literally (case sensitive)
.*?
    matches any character (except for line terminators)
\| 
    matches the character | literally (case sensitive)
\s*
    matches any whitespace character (equal to [\r\n\t\f\v ])

1st Capturing Group ([-+]?[0-9]*\.?[0-9]+)

\s*
    matches any whitespace character (equal to [\r\n\t\f\v ])
\| 
    matches the character | literally (case sensitive)

順便說一句，更安全（更安全匹配）的正則表達式是：

\|\s+Data 1\s+\|\s*?\d+\s*?\|\s*?\d+\s*?\|\s*?\d+\s*?\|\s*([-+]?[0-9]*\.?[0-9]+)\s*\|