XPath 1.0 exclusive or node-set expression - xpath

Question

What I need doesn't quite seem to match what other articles of a similar title are about.
I need, using Xpath 1, to be able to get node a, or node b, excusively, in that order.
That is, node a if it exists, otherwise, node b.
an xpath expression such as :
expression | expression
will get me both in the case they both exist. that is not what I want.
I could go:
(expression | expression)[last()]
Which does in fact gget me what I need (in my case), but seems to be a bit inefficient, because it will evaluate both sides of the expression before the last result is selected.
I was hoping for an expression that is going to stop working once the left side succeeds.
A more concrete example of XML
<one>
<two>
<three>hello</three>
<four>bye</four>
</two>
<blahfive>again</blahfive>
</one>
and the xpath that works (but inefficient):
(/one/*[starts-with(local-name(.), 'blah')] | .)[last()]
To be clear, I would like to grab the immediate child node of 'one' which starts with 'blah'. However, if it doesn't exist, I would like only the current node.
If the 'blah' node does exist, I do not want the current node.
Is there a more efficient way to achieve this?

Answer 1

I need, using Xpath 1, to be able to get node a, or node b,
excusively, in that order. That is, node a if it exists, otherwise,
node b.
an xpath expression such as :
expression | expression
will get me both in the case they both exist. that is not what I want.
I could go:
(expression | expression)[last()]
Which does in fact gget me what I need (in my case),
This statement is not true.
Here is an example. Let us have this XML document:
<one>
<a/>
<b/>
</one>
Expression1 is:
/*/a
Expression2 is:
/*/b
Your composite expression:
(Expression1 | Expression2)[last()]
when we substitute the two expressions above is:
(/*/a | /*/b)[last()]
And this expression actually selects b -- not a -- because b is the last of the two in document order.
Now, here is an expression that selects just a if it exists, and selects b only if a doesn't exist -- regardless of document order:
/*/a | /*/b[not(/*/a)]
When this expression is evaluated on the XML document above, it selects a, regardless of its document order -- try swapping in the XML document above the places of a and b to confirm that in both cases the element that is selected is a.
To summarize, one expression that selects the wanted node regardless of any document order is:
Expression1 | Expression2[not(Expression1)]
Let us apply this general expression in your case:
Expression1 is:
/one/*[starts-with(local-name(.), 'blah')]
Expression2 is:
self::node()
The wanted expression (after substituting Expression1 and Expression2 in the above general expression) is:
/one/*[starts-with(local-name(.), 'blah')]
|
self::node()[not(/one/*[starts-with(local-name(.), 'blah')])]