* Revert parts of [28773] and [28727] and [29748].
* Do not crash PHP. Make the shortcode quantifier possessive to avoid backtracks.
* Reduce backtracking in long HTML comments by 100x.
* Do not ignore unclosed HTML comments.
* Do not break unregistered shortcodes, e.g. `[hello attr="value"]`.
* Do not break HTML in shortcode attributes, e.g. `[hello attr="<"]`.
* Do not match for shortcodes when there is extra whitespace, e.g. `[ hello ]`.
* Add unit tests to show #12690 was not fully resolved.
* Tested PHP 5.2.4, 5.2.13, 5.4.32, and 5.5.8.
Adds/modifies unit tests.
Props miqrogroove.
See #29557.
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