问题
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?
回答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.
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
回答2:
It seems like <>^&| are also valid characters, as long as they are properly escaped:
C:\>set ^<^>^^^&^|=xyz
C:\>echo %<>^&|%
xyz
回答3:
seems that the only prohibited character is equal ("=") sign - https://msdn.microsoft.com/en-us/library/windows/desktop/ms682653(v=vs.85).aspx
The name of an environment variable cannot include an equal sign (=).
来源：https://stackoverflow.com/questions/20635695/what-are-valid-characters-for-windows-environment-variable-names-and-values