A criteria is a string in the form of `[class="regex.*" title="str"]`.
It is stored in a struct with a list of *tokens* which is a
attribute/value pair (stored as a `crit_token` struct). Most tokens will
also have a precompiled regex stored that will be used during criteria
matching.
for_window command: When a new view is created its metadata is tested
against all stored criteria, and if a match is found the associated
command list is executed.
Unfortunately some metadata is not available in sway at the moment
(specifically `instance`, `window_role` and `urgent`). Any criteria
string that tries to match an unsupported attribute will fail.
(Note that while the criteria code can be used to parse any criteria
string it is currently only used by the `for_window` command.)
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