Docs feedback: masked variables is not clear what "unescaped characters" means

Problem to solve

The CE/EE docs on masked variables says

The value must not have escape characters.

But, it does not provide a list of those characters or even indicate the parsing context, in any way.

Further details

Proposal

The most work for you, but the best outcome for me would be a static list of all of the disallowed escape characters. If it's all I can get, then at least describe what system of escape characters we're talking about (POSIX? Ruby? Rails?) and I can go look 'em up (or you can provide a link).

Who can address the issue

Other links/references