Using "//" And ".//" Expressions In XPath XML Search Directives In ColdFusion

Years ago, in my introduction to xmlSearch() and XPath in ColdFusion, I talked about the / and // expressions. As far as describing node locations, both of these expressions have a different meaning depending on where they are located within the greater XPath value. Up until now, however, I had never thought too deeply about the use of these expressions in an xmlSearch() call that didn't originate at the root XML element. As is turns out, sub-tree searching has few interesting expression-based caveats.

Before we get into the sub-tree searching, let's just quickly recap how / and // work in relation to the root node. When / is used at the beginning of a path:

/a

... it will define an absolute path to node "a" relative to the root. As such, in this case, it will only find "a" nodes at the root of the XML tree.

When // is used at the beginning of a path:

//a

... it will define a path to node "a" anywhere within the XML document. As such, in this case, it will find "a" nodes located at any depth within the XML tree.

These XPath expressions can also be used in the middle of an XPath value to define ancestor-descendant relationships. When / is used in the middle of a path:

/a/b

... it will define a path to node "b" that is a direct descendant (ie. a child) of node "a".

When // used in the middle of a path:

/a//b

... it will define a path to node "b" that is any descendant of of node "a".

Ok, so that brings us back up to speed with how / and // expressions can be used with root-relative XPath values; but, what about when we have an existing reference to a sub-node of an XML tree? How do these expressions work?

As it turns out, they work exactly the same - but, in order to define a "relative" path you have to use dot-notation. ColdFusion's xmlSearch() function takes a node and an XPath value. That XPath value adheres to the same rules thats we discussed above. And, if you want to use the "in the middle of a path" concept, you have to start your XPath value with a "." (period).

So, given some sub-node, "x", the XPath:

//y

... will still find any node, "y", located anywhere within the XML tree. But, the XPath:

.//y

... will find any node, "y", that is a descendant of the node "x." In other words, preceding the "//" expression with a "." tells the XML search engine to execute the search relative to the current node reference.

To illustrate this concept, take a look the following code. In it, we're going to grab a sub-node reference and then perform a few searches using "//" and ".//" notation.

When we run the above code, comparing the use of "//" and ".//" XPath expressions in node-relative xmlSearch() calls, we get the following output:

As you can see, when performing a node-relative xmlSearch() call in ColdFusion, you have to prefix the XPath value with "." to make the expression "//" location-relative; otherwise, the given XPath value will operate as if you were searching from the root node.

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