Description:
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From manual page: http://www.php.net/regexp.reference.assertions
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"An assertion is a test on the characters following or preceding the current matching point that does not actually consume any characters. The simple assertions coded as \b, \B, \A, \Z, \z, ^ and $ are DESCRIBED ABOVE". 

ABOVE WHERE? This is the beginning of the page, there's no "above".
This probably means "somewhere in one of the earlier sections", which are about a dozen, so a link must be given (or the name of the section indicated), otherwise one has to look through all those previous sections to find the one where \b, \B etc are described.