SWI-Prolog -- xpath/3

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Availability:


    :- use_module(
    
     library(xpath)
    
    ).

(can be autoloaded )

[nondet] xpath ( +DOM, +Spec, ?Content )

Match an element in a DOM structure. The syntax is inspired by XPath, using () rather than

[]

to select inside an element. First we can construct paths using / and //: The Terms above are of type callable . The functor specifies the element name. The element name’*’refers to any element. The name


    self

refers to the top-element itself and is often used for processing matches of an earlier xpath/3 query. A term NS:Term refers to an XML name in the namespace NS. Optional arguments specify additional constraints and functions. The arguments are processed from left to right. Defined conditional argument values are:

index(?Index)

True if the element is the Index-th child of its parent, where 1 denotes the first child. Index can be one of:

Var

Var is unified with the index of the matched element.

last

True for the last element.

last - IntExpr

True for the last-minus-nth element. For example,


    last-1

is the element directly preceding the last one.

IntExpr

True for the element whose index equals IntExpr .

Integer

The N-th element with the given name, with 1 denoting the first element. Same as


    index(Integer)

last

The last element with the given name. Same as


    index(last)

last - IntExpr

The IntExpr-th element before the last. Same as


    index(last-IntExpr)

self

Evaluate to the entire element

content

Evaluate to the content of the element (a list)

text

Evaluates to all text from the sub-tree as an atom

text(As)

Evaluates to all text from the sub-tree according to As , which is either


    atom


    string

normalize_space


    text

, but uses normalize_space/2 to normalise white-space in the output

number

Extract an integer or float from the value. Ignores leading and trailing white-space

@ Attribute

Evaluates to the value of the given attribute. Attribute can be a compound term. In this case the functor name denotes the attribute and arguments perform transformations on the attribute value. Defined transformations are:

number

Translate the value into a number using xsd_number_string/2 from


    library(sgml)

integer


    number

, but subsequently transform the value into an integer using the round/1 function.

float


    number

, but subsequently transform the value into a float using the float/1 function.

atom

Translate the value into a Prolog atom. Note that an atom is normally the default, so


    @href

and


    @href(atom)

are equivalent. The SGML parser can return attributes as strings using the


    attribute_value(string)

option.

string

Translate the value into a Prolog string.

lower

Translate the value to lower case, preserving the type.

upper

Translate the value to upper case, preserving the type.

Left = Right

Succeeds if the left-hand unifies with the right-hand. If the left-hand side is a function, this is evaluated. The right-hand side is never evaluated, and thus the condition


    content = content

defines that the content of the element is the atom


    content

. The functions


    lower_case

and


    upper_case

can be applied to Right (see example below).

contains(Haystack, Needle)

Succeeds if Needle is a sub-string of Haystack.

XPath

Succeeds if XPath matches in the currently selected sub- DOM . For example, the following expression finds an

h3

element inside a

div

element, where the

div

element itself contains an

h2

child with a


    strong

child. Match the last cell of each tablerow in DOM . This example illustrates that a result can be the input of subsequent xpath/3 queries. Using multiple queries on the intermediate TR term guarantee that all results come from the same table-row: xpath(DOM, //tr, TR), xpath(TR, /td(last), TD) Suppose we have a table containing rows where each first column is the name of a product with a link to details and the second is the price (a number). The following predicate matches the name, URL and price: product(DOM, Name, URL, Price) :- xpath(DOM, //tr, TR), xpath(TR, td(1), C1), xpath(C1, /self(normalize_space), Name), xpath(C1, a(@href), URL), xpath(TR, td(2, number), Price). xpath(DOM, //div, DIV), being a syntax error until I realized div is an operator. xpath(DOM, //'div', DIV),