what does this xpath mean?
//p[not(ancestor::*[3])]
//table[ancestor::*[1][self::p] or ancestor::*[2][self::p]]
tr/td//a[anc开发者_Python百科estor::*[1`][self::td] or ancestor::*[2][self::td]]
// # from the root node, look at all descendants
p[ # select nodes of type <p>, who have…
not(ancestor::*[3]) # …no ancestor 3 levels up
] #
// # from these nodes, select descendants
table[ # of type <table>, who have…
ancestor::*[1][self::p] # …a <p> as their direct ancestor
or # or
ancestor::*[2][self::p] # …a <p> as their second ancestor
] #
# syntax error, this should be a location step
tr # …select all nodes of type <tr>
/ # from their children…
td # …select all nodes of type <td>
// # from their descendants…
a[ # …select all nodes of type <a>, who have
ancestor::*[1][self::td] # …a <td> as their direct ancestor
or # or
ancestor::*[2][self::td] # …a <td> as their second ancestor
]
Or, expressed in HTML:
<html>
<body>
<p>
<table>
<tr>
<td>
<a title="These would be selected." />
</td>
</tr>
</table>
</p>
</body>
</html>
The whole XPath does not make too much sense anyway. It goes without saying that <p><table> is invalid HTML.
Lets break this down:
//p[not(ancestor::*[3])]
Selects all p tags who do not have a 3rd ancestor.
In those:
//table[ancestor::*[1][self::p] or ancestor::*[2][self::p]]
It selects all table tags whose first or second ancestor is a p tag.
Then:
tr/td//a[ancestor::*[1`][self::td] or ancestor::*[2][self::td]]
This isn't entirely correct (there should be a / at the start). However, it goes down the tr/td//s to select all a tags whose first or second ancestor is a td tag.
All and all, it is very convoluted and could probably be a lot easier to achieve with some id attributes defined in the relevant places.
The XPath expression you specified:
//p[not(ancestor::*[3])]
//table[ancestor::*[1][self::p] or ancestor::*[2][self::p]]
tr/td//a
isn't syntactically legal -- it is missing a / before the tr.
The corrected XPath expression:
//p[not(ancestor::*[3])]
//table[ancestor::*[1][self::p] or ancestor::*[2][self::p]]
/tr/td//a
was provided as answer to this question.
As explained in the linked (above) answer, the meaning is:
This selects all a elements whose parent or grand-parent is td, whose parent is a tr, whose parent is a table, whose parent or grandparent is a p that has less than 3 ancesstor - elements
The OP wanted a way to get the a elements that can be located under a p buried not deeper than 3 levels beneath the root of the document, then under a table/tr/td where the table is buried at a level not greater than 3 from the p.
Certainly, wanting to select such nodes may not seem too-meaningful, but we are in no position to judge anyone's needs and requirements.
The amazing fact is that XPath is so powerful to satisfy even such requirements.
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