Question

I am writing an application in C# that will need to find placeholders in URLs and/or filenames, and substitute in a value, much like this: C:\files\file{number} => C:\files\file1 Unfortunately for that example, curly braces are allowed in file names and URLs.

Can anyone please suggest some characters that I can use to denote placeholders in files and URLs? Thank you!

Answer 1

Windows rather helpfully tells you what characters aren’t allowed in a filename, when you try to use on of them:

A filename cannot contain any of the following characters:
\ / : * ? ” < > |

See this support article for more information, including the list of allowed characters.

Characters that are valid for naming
files, folders, or shortcuts include
any combination of letters (A-Z) and
numbers (0-9), plus the following
special characters:

^   Accent circumflex (caret)
&   Ampersand
'   Apostrophe (single quotation mark)
@   At sign
{   Brace left
}   Brace right
[   Bracket opening
]   Bracket closing
,   Comma
$   Dollar sign
=   Equal sign
!   Exclamation point
-   Hyphen
#   Number sign
(   Parenthesis opening
)   Parenthesis closing
%   Percent
.   Period
+   Plus
~   Tilde
_   Underscore

As for URLs, see section 2.2 of RFC 1738 for a description of allowed characters:

Thus, only alphanumerics, the
special characters “$-_.+!*'(),”, and
reserved characters used for their reserved purposes may be used unencoded within a URL.

…also of interest, from the same section:

Characters can be unsafe for a number
of reasons. The space character is
unsafe because significant spaces may
disappear and insignificant spaces may
be introduced when URLs are
transcribed or typeset or subjected to
the treatment of word-processing
programs. The characters “<” and “>”
are unsafe because they are used as
the delimiters around URLs in free
text; the quote mark (“””) is used to
delimit URLs in some systems. The
character “#” is unsafe and should
always be encoded because it is used
in World Wide Web and in other systems
to delimit a URL from a
fragment/anchor identifier that might
follow it. The character “%” is
unsafe because it is used for
encodings of other characters. Other
characters are unsafe because gateways
and other transport agents are known
to sometimes modify such characters.
These characters are “{“, “}”, “|”,
“\”, “^”, “~”, “[“, “]”, and “`”.

All unsafe characters must always be encoded within a URL.

It looks like the double-quote and angle bracket characters ("<>) are good options.