The bufnr() function can find buffers (and return its number). Like bufname(), this can take the queried buffer name in several forms (cp. :help bufname()):
A full match is preferred, otherwise a match at the start, end or middle of the buffer name is accepted. If you only want a full match then put "^" at the start and "$" at the end of the pattern.
So, one example would be
:echo bufnr('^C:\path\to\file.txt$') Also, you can convert between relative and absolute paths via the fnamemodify() function.