How do we, in the definitive way, convert C source codes into C++ having stdint.h, stdlib.h, string.h, stdio.h, inttypes.h, pthread.h, done mostly by regex find-replace?
just compile the c code as-is into the project if that is possible. If the code works, use it, anything else is wasting time and time is money.
you can convert it to C++ 1990 procedural format with minimal work... change all the memory to new/delete, change printf to cout, and so on, a few basic changes and fix the header files and std:: namespace issues. If you want c++ 17 from C using OSO design, it is a total rewrite, there is NO way to convert it with just replace logic. There isnt much point in turning it into 1990s era C++. Why bother? You can go somewhere between these two extremes, procedural c++17 and swap arrays to vectors etc, but doing that with replacements is still a no-go.
If the C code compiles and works as wanted why change it?
As a learning experience, great idea.
But doing the conversion will require as much artistry and technical skill as creating an art masterpiece. Each converted program will be a work of art, hand crafted.