Re: [xml] How could I understand the slightly difference of the parsing process of E1/E2[E3] and E1[E2][E3]?

[Bruce R Miller <bruce miller nist gov>]

Ming Chen wrote:
> Hi Experts,
> Recently a colleague and I have disagreed on the parsing process of the 
> E1/E2[E3], where E3 has a numeric type. Here is the example XML file:
> <?xml version="1.0" encoding="UTF-8"?>
> <xml>
> <table>
> <rec id="1">
> <para type="error" position="11"/>
> <para type="warning" position="12"/>
> <para type="warning" position="13"/>
> </rec>
> <rec id="2">
> <para type="warning" position="21"/>
> <para type="warning" position="22"/>
> <para type="warning" position="23"/>
> </rec>
> <rec id="3">
> <para type="info" position="31"/>
> <para type="warning" position="32"/>
> <para type="warning" position="33"/>
> </rec>
> </table>
> </xml>
> For XPath expression "//rec/para[1]", xmllint.exe outputs:
> <para type="error" position="11"/><para type="warning" 
> position="21"/><para type="info" position="31"/>
> While my colleague said that the output should be: <para type="error" 
> position="11"/>

Your colleague is is interpreting the xpath expression as 
( //rec/para ) [1]
rather than
//rec / (para[1])

See section 2.1 of the XPath (1.0) spec; The predicate,
in this case [1], is part of the "step".
So //rec selects a set of <rec> nodes, then for each, the
next step "para[1]" is applied and the union is formed.
Not that the predicate [1] is applied to the union.

...

> So, what's the nice distinction between E1/E2[E3] and E1[E2][E3]. 

The first xpath has 2 steps, where the 2nd step has one predicate.
The second xpath has 1 step which has 2 predicates.
When there are multiple predicates, they are applied
left to right as if they were successive filters.