Safe file prefix to be on top in a list

Here is the list of possible candidates:

Oct Dec Hex  Name
040  32 0x20 space
041  33 0x21 !, exclamation mark
042  34 0x22 ", double quote
043  35 0x23 #, hash
044  36 0x24 $, dollar
045  37 0x25 %, percent
046  38 0x26 &, ampersand
047  39 0x27 ', quote
050  40 0x28 (, open parenthesis
051  41 0x29 ), close parenthesis
052  42 0x2a *, asterisk
053  43 0x2b +, plus
054  44 0x2c ,, comma
055  45 0x2d -, minus
056  46 0x2e ., full stop
057  47 0x2f /, oblique stroke

/ is reserved by FS. Space is hard to see. Quoting characters require awful quoting.

! is used for history expansion by shells in interactive mode.

# at the beginning of word is comment syntax in shell.

$ used for variable substitution in shells.

& is command separator in Windows CMD and control operator in shells.

( and ) have special meaning in shells along with *.

*, ., + are special in regex.

- is used as marker of comand argument. Usage of special agreement to supply file list after -- or prefixing like ./- is common technique to allow - as file prefix.

% should be quoted in URL and have special meaning in Windows CMD and environment variables.

Dot is safe as prefix in file name but solely . and .. have special meaning for FS and it should be qouted in regex.

The winner is comma , although it is hard to distinguish from dot sign. You may see comma as prefix in a Lisp special backquote syntax.

Some tools uses comma as field separator. For example in Ant build tools this:

<fileset dir="." includes="**/*,*.xml"></fileset>

should be replaced with:

<fileset dir=".">
    <include name="**/*,*.xml" />
</fileset>

because , have special meaning when used in includes attribute.

Another pretty safe prefix is a plus sign except that it requires escaping in regex.

GNU Coreutils have utility that checks file names for safety:

$ cd $DIR
$ pathchk --portability **/*

https://en.wikipedia.org/wiki/Filename#Reserved_characters_and_words: List of reserved characters prohibited in file names.
https://support.apple.com/en-us/HT202808: OS X: Cross-platform filename best practices and conventions.
https://msdn.microsoft.com/en-us/library/windows/desktop/aa368590.aspx: Filename on Windows.
http://unix.stackexchange.com/questions/155793/is-it-correct-to-use-certain-special-characters-when-naming-filenames-in-linux: Is it correct to use certain special characters when naming filenames in Linux?
http://superuser.com/questions/358855/what-characters-are-safe-in-cross-platform-file-names-for-linux-windows-and-os: What characters are safe in cross-platform file names for Linux, Windows and OS-X.

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