What are valid characters for Windows environment variable names and values?

Question

After some reseach, I found out that length limits for names are 255 and for values 32767 characters.

But which characters are allowed for names?
And which characters are allowed for values?

Answer 1

About variable values: you can use most characters as variable values, including white space. If you use the special characters <, >, |, &, or ^, you must precede them with the escape character (^) or quotation marks. If you use quotation marks, they are included as part of the value because everything following the equal sign is taken as the value.

http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/ntcmds_shelloverview.mspx?mfr=true

Check section "Setting environment variables".

About variable names: in my opinion, for best compatibility with every application, you should limit yourself to letters, numbers, underscore (_) and minus (-).

I'm quite sure that all POSIX valid characters for files are ok, but I did not found any evidence of this.

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Concerning variable names names we need to also accept parenthesis since %ProgramFiles(x86)% is a well-known envar. From my experiments it seems that in addition to letters and digits characters, these characters are valid _(){}[]$*+-\/"#',;.@!? and these characters are not valid %<>^&|=:.

I didn't do an exhaustive search but just tested most common non alphanumeric characters.

And just for the fun of it you can name an envar %_(){}[]$*+-\/"#',;.@!?%:

C:\>set _(){}[]$*+-\/"#',;.@!?=xyz  C:\>echo %_(){}[]$*+-\/"#',;.@!?% xyz 

Answer 2

It seems like <>^&| are also valid characters, as long as they are properly escaped:

C:\>set ^<^>^^^&^|=xyz  C:\>echo %<>^&|% xyz