std::string string1 = "xjdfhfakdjs%54k34k.-jk34";

std::string the_prefix_you_want = string1.substr(0, string1.find("%"));

#include <windows.h>
void main ()
{
    int i;
    wchar_t searchString[100];
    wchar_t * stringReturn;
    memset(searchString, L'', sizeof(stringReturn));
    wcscpy_s(searchString, 100, L"String\ to search");
    stringReturn = wcschr (searchString, '\');
    if (stringReturn)
    {
        for (i = 0; i < (int)(stringReturn - searchString); i++) stringReturn[i] = searchString[i];
        stringReturn[i] = L'';
        wcscpy_s(searchString, 100, stringReturn);
    }
}