To see what I mean, run:

mkdir colonic\\\:folder
src/nano colonic\\\:folder/somefile

See that nano reports: "[ Directory 'colonic:folder' does not exist ]".  The colon-parsing code has elided the backslash before the colon in the folder name.  :/

Of course, having a backslash in the name of a file or directory is absurdly rare, but it is possible, and nano shouldn't trip up the user who has files with such names.  Nor users with file names that end in a colon plus some digits, which are seemingly somewhat common on some systems (even though they are mostly log files and the user is unlikely to want to edit them).

So... for one thing, the colon parsing should be behind an option, so that unsuspecting users will not experience any adverse effects.

Second, nano should parse the given filename in reverse, checking from the tail whether there are digits preceded by a colon (preceded possibly by more digits plus a colon), so as to avoid trying to parse colons that cannot possibly specify a line number or column number.

And third, the eliding of a backslash before a colon should probably be removed entirely, and another mechanism for overriding should be found -- the simple use of +1 before the filename could disable colon parsing, for example.