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aba,abthenabut then for that a, there is no match for the contained subexpression within the string matched by the containing subexpression, so \2 won't match. Is there no match for the contained subexpression or is it an empty match? <asterisk> it shall match what zero or more consecutive occurrences of the BRE would match\(b*\)whereb*would match an empty string, its\(b\)*, wherebdoesn't match.