This adds a general, platform independent example before the more detailed
example.
Also removes details about the reason of the platform dependency of some
non-standard regular expressions, and replaces it with a hyperlink to the related
Zsh document.
In BSD-based platform such as macOS, the metacharacters with leading "\" are not supported.
This adds comments for users with those OS, to avoid confusing results.
Also changes the wrong associative array name in the example(s).
