Relations and Subrelations

As mentioned in the section Fundamentals of Pattern Specification, an attribute of a specific relation (i.e. of a specific element) can be a relation with its own attributes. DEXTL provides support to process these cases through the hierarchical arrangement of different DEXTL elements within the same program.

When this occurs, it is said that the subrelation associated with the attribute is a “child” of the relation that contains the attribute.

For the system to be able to extract tuples from a subrelation a new element must be defined for them, as a subelement of the element that extracts tuples from the parent relation. This is specified by including it between the symbols ‘{‘ and ‘}’. The name of the new subelement must be specified at the start of the subelement, with the notation ‘NAME=”NAMEELEMENT”’, where NAMEELEMENT is the name assigned to the element. After the name of the subelement, a FROM clause can be declared as usual, followed by the pattern of the subelement.

In addition, a pattern that demarcates the end of the subrelation must be indicated in order for the system to recognize when it should stop the search for tuples of the subrelation and return to the search for tuples from the relation or superior element. For this a pattern has to be provided inside a TO clause; it constitutes the end tag of the subrelation. As a regular TO pattern, it must be inserted between the symbols ‘<’ and ‘>’ immediately before the end tag of the subelement ‘‘}’. If this TO pattern is not included, it is implicitly assumed that the subrelation has to be searched until the end of the current document. An important detail is that the text that causes the TO pattern to be matched is not discarded: once the subelement has been abandoned the text is returned to the input and will be considered again in the next search of the parent element.

Different format tags can be used for the pattern of each relation or subrelation. In fact, each pattern contained in a block between ‘{‘ and ‘‘}’ can specify its own format tagset by putting the expression ‘TAGSET=”NAME”’ before the pattern definition, where NAME represents the name of the format tagset to be used. It is important to highlight that the pattern specified by the TO pattern of a subrelation uses the tagset defined for the pattern that is being analyzed at each moment in time (this is due to the fact that each pattern of the element can have a different tagset).

Example: Consider an electronic video shop represented by the relation R={TITLE, ACTORS, DIRECTOR, GENRE, SUMMARY, EDITION{DESCRIPTION, FORMAT, PRICE}}.

Output from an Electronic Video Shop

This figure shows an example of a fragment of a Web page in which a tuple of R is represented graphically.

To construct this specification a tagset will be defined containing only three format tags: the BR, ENDTD and ENDTABLE.

ENDTABLE represents the end of a table in HTML. Thus, ENDTABLE=”</TABLE”[^\>]*”>”.

The DEXTL program that would be able to extract tuples with this format is shown below:

{ NAME="R"
 "Starring:" :ACTORS BR
 "Director:" :DIRECTOR BR
 "Synopsis:" :SUMMARY BR
 "Genre:" :GENRE BR
   FROM "Description:" ENDTD "Format:" ENDTD "List:" ENDTD "Price:" ENDTD "Availability:"

HTML of “Output from an Electronic Video Shop” without the irrelevant tags shows the HTML code associated with Output from an Electronic Video Shop after deleting all those HTML tags ignored by the DEXTL program, as they did not match any of the format tag definitions. These constitute most of the document.

HTML of “Output from an Electronic Video Shop” without the irrelevant tags
Small Time Crooks (2000)<BR>
Starring: Woody Allen, Tracey Ullman<BR>
Director: Woody Allen<BR>
Synopsis: Manic crime comedy about an affable ex-con (Allen) cooking up a cunning bank robbery while working in a restaurant.<BR>
Runtime: 95 minutes<BR>
MPAA Rating: PG<BR>
Genre: Comedy<BR>

</TD> Description:</TD>Format:</TD>List:</TD>Price:</TD>Availability:</TD>
Small Time Crooks (Widescreen)</TD>DVD</TD>$26.99</TD>$20.99</TD>Pre-Order</TD>
Small Time Crooks</TD>VHS</TD>$14.99</TD>$10.49</TD>Pre-Order</TD>
Small Time Crooks (Spanish Subtitles)</TD>VHS</TD>$14.99</TD>$10.49</TD>Pre-Order</TD>

The document analysis will now operate in the following manner:

  • Until the beginning of the subrelation is reached it operates in the same manner as that seen before.

  • When it is time for a specific tuple extraction process to search for tuples in the subrelation (because it matches the pattern defined in the FROM clause of the subrelation):

    • It executes the change of tagset, if it is different to that of the parent element (in this case this is not so).

    • It begins to simultaneously search for the tuple pattern in the subrelation main patterns (to look for data to extract) and the subrelation end tag pattern (to check if the end of the subrelation search area has been reached).

      • The search for tuples in the subrelation is developed in the same manner as the search for tuples in a parent element.
      • When the subrelation end tag is found, the group of tuples extracted from the subrelation becomes the value of the attribute corresponding to the tuple of the parent element. In addition, the text that triggered the detection of the end tag is returned to the beginning.
    • When processing of the subelement is complete, the search for the attributes that are missing from the tuple of the parent element proceeds.

There is one restriction to the use of subrelations, which is as follows: when an element of a DEXTL program has a subrelation as a first attribute (i.e. a non-atomic attribute), the system has to match at least one occurrence of a subrelation before being able to match the pattern TO. Therefore, the number of occurrences of subelements that constitute the first component of an element will be between 1 and n instead of between 0 and n.