Section: User Contributed Perl Documentation (3)
Updated: 2021-01-27
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Text::BibTeX::Value - interfaces to BibTeX values and simple values  


   use Text::BibTeX;

   $entry = Text::BibTeX::Entry->new;

   # set the 'preserve_values' flag to 1 for this parse
   $entry->parse ($filename, $filehandle, 1);

   # 'get' method now returns a Text::BibTeX::Value object 
   # rather than a string
   $value = $entry->get ($field);

   # query the `Value' object (list of SimpleValue objects)
   @all_values = $value->values;
   $first_value = $value->value (0);
   $last_value = $value->value (-1);

   # query the simple value objects -- type will be one of BTAST_STRING,
   use Text::BibTex (':nodetypes');   # import "node type" constants
   $is_macro = ($first_value->type == BTAST_MACRO);
   $text = $first_value->text;



The "Text::BibTeX::Value" module provides two classes, "Text::BibTeX::Value" and "Text::BibTeX::SimpleValue", which respectively give you access to BibTeX ``compound values'' and ``simple values''. Recall that every field value in a BibTeX entry is the concatenation of one or more simple values, and that each of those simple values may be a literal string, a macro (abbreviation), or a number. Normally with "Text::BibTeX", field values are ``fully processed,'' so that you only have access to the string that results from expanding macros, converting numbers to strings, concatenating all sub-strings, and collapsing whitespace in the resulting string.

For example, in the following entry:

     author = "Homer Simpson" # and # "Ned Flanders",
     title = {Territorial Imperatives in Modern Suburbia},
     journal = jss,
     year = 1997

we see the full range of options. The "author" field consists of three simple values: a string, a macro ("and"), and another string. The "title" field is a single string, and the "journal" and "year" fields are, respectively, a single macro and a single number. If you parse this entry in the usual way:

   $entry = Text::BibTeX::Entry->new($entry_text);

then the "get" method on $entry would return simple strings. Assuming that the "and" macro is defined as " and ", then

   $entry->get ('author')

would return the Perl string "Homer Simpson and Ned Flanders".

However, you can also request that the library preserve the input values in your entries, i.e. not lose the information about which values use macros, which values are composed of multiple simple values, and so on. There are two ways to make this request: per-file and per-entry. For a per-file request, use the "preserve_values" method on your "File" object:

   $bibfile = Text::BibTeX::File->new($filename);
   $bibfile->preserve_values (1);

   $entry = Text::BibTeX::Entry->new($bibfile);
   $entry->get ($field);        # returns a Value object

   $bibfile->preserve_values (0);
   $entry = Text::BibTeX::Entry->new($bibfile);
   $entry->get ($field);        # returns a string

If you're not using a "File" object, or want to control things at a finer scale, then you have to pass in the "preserve_values" flag when invoking "read", "parse", or "parse_s" on your "Entry" objects:

   # no File object, parsing from a string
   $entry = Text::BibTeX::Entry->new;
   $entry->parse_s ($entry_text, 0);  # preserve_values=0 (default)
   $entry->get ($field);        # returns a string

   $entry->parse_s ($entry_text, 1);
   $entry->get ($field);        # returns a Value object

   # using a File object, but want finer control
   $entry->read ($bibfile, 0);  # now get will return strings (default)
   $entry->read ($bibfile, 1);  # now get will return Value objects

A compound value, usually just called a value, is simply a list of simple values. The "Text::BibTeX::Value" class (hereinafter abbreviated as "Value") provides a simple interface to this list; you can request the whole list, or an individual member of the list. The "SimpleValue" class gives you access to the innards of each simple value, which consist of the type and the text. The type just tells you if this simple value is a string, macro, or number; it is represented using the Perl translation of the ``node type'' enumeration from C. The possible types are "BTAST_STRING", "BTAST_NUMBER", and "BTAST_MACRO". The text is just what appears in the original entry text, be it a string, number, or macro.

For example, we could parse the above entry in ``preserve values'' mode as follows:

   $entry->parse_s ($entry_text, 1);   # preserve_values is 1

Then, using the "get" method on $entry would return not a string, but a "Value" object. We can get the list of all simple values using the "values" method, or a single value using "value":

   $author = $entry->get ('author');   # now a Text::BibTeX::Value object
   @all_values = $author->values;      # array of Text::BibTeX::SimpleValue
   $second = $author->value (1);       # same as $all_values[1]

The simple values may be queried using the "Text::BibTeX::SimpleValue" methods, "type" and "text":

   $all_values[0]->type;               # returns BTAST_STRING
   $second->type;                      # returns BTAST_MACRO

   $all_values[0]->text;               # "Homer Simpson"
   $second->text;                      # "and" (NOT the macro expansion!)

   $entry->get ('year')->value (0)->text;   # "1997"



Normally, you won't need to create "Value" or "SimpleValue" objects---they'll be created for you when an entry is parsed, and returned to you by the "get" method in the "Entry" class. Thus, the query methods ("values" and "value" for the "Value" class, "type" and "text" for "SimpleValue") are probably all you need to worry about. If you wish, though, you can create new values and simple values using the two classes' respective constructors. You can also put newly-created "Value" objects back into an existing "Entry" object using the "set" entry method; it doesn't matter how the entry was parsed, this is acceptable anytime.  

Text::BibTeX::Value methods

new (SVAL, ...)
Creates a new "Value" object from a list of simple values. Each simple value, SVAL, may be either a "SimpleValue" object or a reference to a two-element list containing the type and text of the simple value. For example, one way to recreate the "author" field of the example entry in ``DESCRIPTION'' would be:

   $and_macro = Text::BibTeX::SimpleValue->new (BTAST_MACRO, 'and');
   $value = Text::BibTeX::Value->new 
      ([BTAST_STRING, 'Homer Simpson'],
       [BTAST_STRING, 'Ned Flanders']);

The resulting "Value" object could then be installed into an entry using the "set" method of the "Entry" class.

values ()
Returns the list of "SimpleValue" objects that make up a "Value" object.
value (NUM)
Returns the NUM'th "SimpleValue" object from the list of "SimpleValue" objects that make up a "Value" object. This is just like a Perl array reference: NUM is zero-based, and negative numbers count from the end of the array.

Text::BibTeX::SimpleValue methods

new (TYPE, TEXT)
Creates a new "SimpleValue" object with the specified TYPE and TEXT. TYPE must be one of the allowed types for BibTeX simple values, i.e. "BTAST_STRING", "BTAST_NUMBER", or "BTAST_MACRO". You'll probably want to import these constants from "Text::BibTeX" using the "nodetypes" export tag:

   use Text::BibTeX qw(:nodetypes);

TEXT may be any string. Note that if TYPE is "BTAST_NUMBER" and TEXT is not a string of digits, the "SimpleValue" object will be created anyways, but a warning will be issued. No warning is issued about non-existent macros.

type ()
Returns the type of a simple value. This will be one of the allowed ``node types'' as described under ``new'' above.
text ()
Returns the text of a simple value. This is just the text that appears in the original entry---unexpanded macro name, or unconverted number. (Of course, converting numbers doesn't make any difference from Perl; in fact, it's all the same in C too, since the C code just keeps numbers as strings of digits. It's simply a matter of whether the string of digits is represented as a string or a number, which you might be interested in knowing if you want to preserve the structure of the input as much possible.)


Text::BibTeX, Text::BibTeX::File, Text::BibTeX::Entry  


Greg Ward <gward@python.net>  


Copyright (c) 1997-2000 by Gregory P. Ward. All rights reserved. This file is part of the Text::BibTeX library. This library is free software; you may redistribute it and/or modify it under the same terms as Perl itself.



Text::BibTeX::Value methods
Text::BibTeX::SimpleValue methods