C++ 正则表达式，未知转义序列 '\.' 警告

Question

我第一次尝试在 C++ 中使用正则表达式，我对转义序列有点困惑。我只是想匹配字符串开头的点。为此，我使用了表达式:“^\\\.”，它有效，但我的编译器 (g++) 生成警告:

warning: unknown escape sequence '\.'
        regex self_regex("^\\\.");
                             ^~

如果我使用例如“^\\.”，它不会生成警告，但该正则表达式与我打算做的不匹配。

我也不明白为什么我必须在“\”中使用三个反斜杠，两个就足够了吗？第一个反斜杠转义了第二个反斜杠，所以我实际上搜索了 .，但它不起作用。有人可以为我澄清一下吗？

代码:

#include <iostream>
#include <dirent.h>
#include <regex>

using namespace std;

int main(void){
    DIR *dir;
    string path = "/Users/-----------/Documents/Bibliothek/MachineLearning/DeepLearning/ConvolutionalNeuralNetworks/CS231n 2016/Assignments/assignment3/assignment3/cs231n";
    regex self_regex("^\\\.+");
    struct dirent *ent;
    dir = opendir(path.c_str());
    if ((dir = opendir(path.c_str())) != NULL){
        while ((ent = readdir(dir)) != NULL){
            if (regex_search(string(ent->d_name),self_regex)){
                cout << "matches regex" << ent->d_name << endl;
            }
            else{
                cout << "does not match regex " << ent->d_name << endl;
            }
        }
        closedir(dir);
    }
    return 0;
}

输出:

matches regex.
matches regex..
matches regex.DS_Store
matches regex.gitignore
does not match regex __init__.py
does not match regex __init__.pyc
does not match regex build
does not match regex captioning_solver.py
does not match regex captioning_solver.pyc
does not match regex classifiers
does not match regex coco_utils.py
does not match regex coco_utils.pyc
does not match regex data_utils.py
does not match regex datasets
does not match regex fast_layers.py
does not match regex fast_layers.pyc
does not match regex gradient_check.py
does not match regex gradient_check.pyc
does not match regex im2col.py
does not match regex im2col.pyc
does not match regex im2col_cython.c
does not match regex im2col_cython.pyx
does not match regex im2col_cython.so
does not match regex image_utils.py
does not match regex image_utils.pyc
does not match regex layer_utils.py
does not match regex layers.py
does not match regex layers.pyc
does not match regex optim.py
does not match regex optim.pyc
does not match regex rnn_layers.py
does not match regex rnn_layers.pyc
does not match regex setup.py

Answer 1

当您在代码中写入字符串文字时:

"^\\\."  

您的编译器将根据 C++ 规则对其进行解析，以生成将在您的可执行文件中使用的字符串。例如，如果遇到 \n，则可执行文件中的字符串将包含换行符。 "\\" 被转换为 "\"，但是你的编译器不知道如何处理 "\."，因为有no such escape sequence在 C++ 中定义。

Escape sequences in which the character following the backslash is not listed (...) are conditionally-supported, with implementation-defined semantics.

因此您要查找的字符串只有两个斜杠:

"^\\."

将被编译器转换成:

"^\."  

这是 regex you're looking for !

备注:例如 GCC 会将未知的转义序列 "\." 转换为 "."，因此 2 或 3 个 bakslashes 实际上会产生相同的结果.

Online demo