Regex explain

Explain this regular expression.

Pattern:
PASTE IT
Flavour or language, if I know it: WHICH.
What it is supposed to match: WHAT.

Five parts.

One sentence. What it matches overall.

Piece by piece. A table: the fragment, what it does, and what it would
match on its own. Include anchors, groups, escapes and every quantifier.

Matches. Five example strings it accepts, chosen to include at least
one that is surprising.

Does not match. Five example strings it rejects, chosen to include the
near misses people expect it to accept. This is the useful half.

Surprises. Where it will not behave as the author intended. Look
specifically at: a dot matching more than expected, a greedy quantifier
running past its intended stop, a missing anchor allowing a partial
match anywhere in the string, a character class that quietly includes
something, and any construct that could backtrack badly on a long
input.

Then one line on flavour. Whether the pattern behaves differently in
another common flavour, and which construct causes the difference.

You cannot execute anything here, so derive every example from the
pattern and say if one is uncertain. Then tell me which of the examples
I should actually run to confirm.