How to do group capture in xpath -

i looking in xpath can do: //foo[@n="$1"]//bar[@n="$2"] can return me $1 , $2. or @ least return me both <foo> , <bar>

here more detail, have xml document:

<xml> <foo> <a n="1"> <b n="1"/> <b n="2"/> </a> </foo> <a n="2"> <b n="1"/> </a> <a n="3"> <b n="1"/> <foo> <b n="2"/> </foo> <b n="3"/> </a> </xml> 

i want generate string base on n attribute in <a> , <b> have xpath: //a[@n]//b[@n] every result back, use: ./@n , ./ancestor::a/@n info want.

this working fine, need more intelligent, because have lot structure this, , need auto generate xpath.

so above example, looking xpath like: //a[@n="$1"]//b[@n="$2"] return me: `(1, 1), (1, 2), (2, 1), (3, 1), (3, 2), (3, 3)

here 1 xpath 1.0 expression selects wanted n attributes:

//a[.//b]/@n | //a//b/@n 

without optimization evaluation of above expression performas @ least 2 complete traversals of xml document.

this xpath 1.0 expression may more efficient:

//*[self::a , .//b or self::b , ancestor::a]/@n 

both expressions can simplified if guaranteed every a has b descendant.

they become, respectively:

//a/@n | //a//b/@n 

and:

//*[self::a or self::b , ancestor::a]/@n 

further simplification possible if guaranteed every a has descendant b , every b has ancestor a.:

//*[self::a or self::b]/@n 

it impossible in single xpath 1.0 expression string values of wanted attributes. 1 needs atributes using 1 of above expressions, on each of selected attributes apply second xpath expression: string().

in xpath 2.0 possible single expression string values of wanted attributes -- appending each of expressions /string(.)

for example, simplest one:

//(a|b)/@n/string(.) 

update:

the op has clarified question. know wants result produced:

(1, 1), (1, 2), (2, 1), (3, 1), (3, 2), (3, 3) 

it isn't possible produce wanted result single xpath 1.0 expression.

the following xpath 2.0 expression produces wanted result:

for $a in //a[@n , .//b[@n]], $b in $a//b[@n] return concat('(', $a/@n, ',', $b/@n, ') ')