Why does ['access' !~ 'no'] filter out ways with access = unknown in Overpass?

Question

If I use this query in Overpass
[out:json];
area[name="Squirrel Hill"];
way(area)
  ['access' !~ 'no'];
out;

then this way (OSM ID 11947323) is not in the result set.
However, if this is my query:
[out:json];
area[name="Squirrel Hill"];
way(area);
out;

The way shows up as expected in the output.
Here you can see that the previously mentioned way does not have the access=no tag.

What is causing this? Is the unknown tag somehow a subset of the no tag?

mmd · Accepted Answer

The filter ['access' !~ 'no']; checks that the tag value for the key access does not match the regular expression "no".
As the string "unknown" contains "no" somewhere in the middle, your way is being filtered out.
Note that regular expressions match anywhere inside a string, unless you explicitly use "^no$" to mark the start and end of a string.

Why does ['access' !~ 'no'] filter out ways with access = unknown in Overpass?

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