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sedand GNUsedon OpenBSD) seem to ignore "If the containing subexpression does not match, or if there is no match for the contained subexpression within the string matched by the containing subexpression, then back-reference expressions corresponding to the contained subexpression shall not match". Because with\1 = a(note: notab) and\2 = b, how can\(a\(b\)*\)*\2matchababwhen "there is no match for the contained subexpression within the string matched by the containing subexpression"?re='\(a\(b\)*\)*\2' string=abab ksh93 -c '[[ $string = ~(G)$re ]]';Gfor BRE)