場景

1.正則表示式在查詢替換字串資料時效率很高, 可以節省很多不必要的查詢程式碼. 特別是對字串分組的查詢, 可以說如果沒有正則表示式,查詢分組裡的字串需要寫很多額外的程式碼，還不一定準確.

2.查詢並替換XML標籤是比較常見的需求, 比如過濾掉HTML標籤裡的標籤, 提取字串內容等.

例子

1.這裡舉例了C++正則庫的分組查詢功能, 一個用於提取特定字串, 一個用於替換字串.

// test-regexp.cpp : 定義控制檯應用程式的入口點。
//

#include "stdafx.h"
#include <regex>
#include <iostream
 
>

std::wstring PickLinkShowContent(const wchar_t* buf1){

    std::wstring buf(buf1);
    std::wregex pattern(L"(<A HREF=\"[^\"]*\">)([^<]*)(</A>)",
        std::regex_constants::ECMAScript|std::regex_constants::icase);
    auto words_begin =
        std::wsregex_iterator(buf.begin(), buf.end(), pattern);
    auto words_end = std::wsregex_iterator();

    std::wstring result;
    int start = 0;
    int end = 0;
    for (std::wsregex_iterator i = words_begin; i != words_end; ++i)
    {
        std::wsmatch match = *i;
        size_t pos = match.position();
        size_t length = match.length();

        std::wstring prev = buf.substr(start,pos-start);

        result.append(prev);
        result.append(match[2]);
        end = pos+length;
        start = pos+length;
    }

    if(end+1 != buf.length()){
        result.append(buf.substr(end));
    }
    return result;
}

std::string ReplaceStr(const char* text,const char* old,const char* _new){
    std::regex pattern(old);
    std::string rep(_new);
    std::string text1(text);
    std::string tmp = std::regex_replace(text1,pattern,rep);
    return tmp;
}

int _tmain(int argc, _TCHAR* argv[])
{
    std::cout << "Test
 
 regex group 1" << std::endl;
    const wchar_t* buf = L"You haven't installed QQPCMgr on this computer. "
        L"Please <a HREF=\"http://www.qq.com\">download</A> and install the latest QQPCMgr first, "
        L"then reboot the program.";
    auto result = PickLinkShowContent(buf);
    std::wcout << result
 
 << std::endl;

    std::cout << "Test regex group 2" << std::endl;
    static const char* buf2 = ""
        "<key>name</key>"
            "<string>百度</string><key>position</key>"
            "<string>2</string>"
        "</dict>"
        "<key>05975BCB-FD95-48A4-A912-37ECACD39871</key>"
        "<dict>"
            "<key>name</key>"
            "<string>必應</string>"
            "<key>url</key>"
            "<string>http://www.bing.com/</string>"
            "<key>position</key>"
            "<string>3</string>";

    auto result2 = ReplaceStr(buf2,"(<key>position</key>[^<]*)(<string>)([0-9]*)(</string>)",
        "$1<integer>$3</integer>");
    std::cout << result2 << std::endl;
    system("pause");
    return 0;
}


````

輸出





<div class="se-preview-section-delimiter"></div>

``` XML
Test regex group 1
You haven't installed QQPCMgr on this computer. Please download and install the
latest QQPCMgr first, then reboot the program.
Test regex group 2
<key>name</key><string>百度</string><key>position</key><integer>2</integer></dic
t><key>05975BCB-FD95-48A4-A912-37ECACD39871</key><dict><key>name</key><string>必
應</string><key>url</key><string>http://www.bing.com/</string><key>position</key
><integer>3</integer>

參考