"I faced a awesome problem"
Which is easily solved using much better** indexing, just as the documentation shows:
** better in the sense simpler, neater, much more efficient, less buggy, and easier to debug than what you are attempting.
"i want to rename variable B to C"
Consider what will happen if the file is named "1+2", or "quit" or anything that is not a valid variable name, e.g. anything ending with file-extension .txt (like your example).
Why do you want to force yourself into writing complex, obfuscated, buggy code?
