英文:
Replace Character Only if it isn't Immediately Preceded By Specified Character
问题
std::string s{ "A;1;\nB;2\nC;3" };
const std::regex re("([^;])\\n");
s = std::regex_replace(s, re, "$1;\n");
这将产生所需的输出：A;1;\nB;2;\nC;3。
英文:
Given a multiple line string A;1;\nB;2\nC;3. It's needed to add a ; before those newline characters that are not immediately preceded by ;. So, the result should be A;1;\nB;2;\nC;3 (a single ; is added after 2). I've tried different regexes but without luck (one of them is presented below). I think the lookbehind assertion would do the job, but it isn't supported.
Question: What regex expression can solve this problem? If none, how to solve it using other methods using pure C++ only (I'm using C++20)?
std::string s{ "A;1;\nB;2\nC;3" };
const std::regex re("(?!;)\\n"); // Produces "A;1;;\nB;2;\nC;3" (redundant ";" after "1").
s = std::regex_replace(s, re, ";\n");
答案1
得分: 1
你可以匹配并捕获字符串的起始位置或除了 ; 字符以外的任何字符，然后捕获换行字符，并使用反向引用替换匹配项：
#include <iostream>
#include <regex>
int main() {
std::string s{ "A;1;\nB;2\nC;3" };
const std::regex re("([^;]|^)(\\n)"); // 生成 "A;1;;\nB;2;\nC;3"（"1" 后多了一个不必要的 ";"）。
s = std::regex_replace(s, re, "$1;$2");
std::cout << s;
return 0;
}
输出：
A;1;
B;2;
C;3
模式详情：
([^;]|^)- 第一组 ($1)：要么是除分号 (;) 字符之外的字符 ([^;])，要么 (|) 是字符串的起始位置 (^)（如果不需要匹配字符串开头的子串，则移除|^）。(\\n)- 第二组 ($2)：换行字符。
英文:
You can match and capture either a string start position or any character other than a ; char, and then capture a newline char, and replace the matches using backreferences:
#include <iostream>
#include <regex>
int main() {
std::string s{ "A;1;\nB;2\nC;3" };
const std::regex re("([^;]|^)(\\n)"); // Produces "A;1;;\nB;2;\nC;3" (redundant ";" after "1").
s = std::regex_replace(s, re, "$1;$2");
std::cout << s;
return 0;
}
See the C++ demo and the regex demo.
Output:
A;1;
B;2;
C;3
Pattern details:
([^;]|^)- Group 1 ($1): either a char other than a semi-colon ([^;]), or (|) a start of the string position (^) (remove|^if there is no need to match substrings at the start of the string)(\\n)- Group 2 ($2): a newline char.
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