The ntext
package provides a binding tag named Ntext for use by text
widgets in place of the default Text binding tag.
Navigation and selection in a text widget require the detection
of words and their boundaries. The word boundary detection
facilities provided by Tcl/Tk through the Text binding tag
are limited because they define only one class of "word" characters
and one class of "non-word" characters. The Ntext binding
tag uses more general rules for word boundary detection, that
define two classes of "word" characters and one class of
"non-word" characters.
The behaviour of Ntext may be configured
application-wide by setting the values of a number of namespace
variables. One of these is relevant to word boundary detection:
::ntext::classicWordBreak
-
0 - (default value) selects Ntext
behaviour, i.e. platform-independent, two classes of word
characters and one class of non-word characters.
-
1 - selects classic Text
behaviour, i.e. platform-dependent, one class of word characters
and one class of non-word characters
-
After changing this value, Ntext 's regexp matching
patterns should be recalculated. See FUNCTIONS for details and advanced
configuration options.
::ntext::tcl_match_wordBreakAfter
::ntext::tcl_match_wordBreakBefore
::ntext::tcl_match_endOfWord
::ntext::tcl_match_startOfNextWord
::ntext::tcl_match_startOfPreviousWord
These variables hold the regexp patterns that are used by
Ntext to search for word boundaries. If they are changed,
subsequent searches are immediately altered. In many situations, it
it unnecessary to alter the values of these variables directly:
instead call one of the functions ::ntext::initializeMatchPatterns, ::ntext::createMatchPatterns.
In the Text binding tag one can change the search rules
by changing the values of the global variables tcl_wordchars and tcl_nonwordchars. The equivalent operation in the
Ntext binding tag is to call ::ntext::createMatchPatterns with appropriate
arguments.
If a simple regexp search should prove insufficient, the
following functions (analogous to the Tcl/Tk core's tcl_wordBreakAfter etc) may be replaced by the
developer:
ntext::new_wordBreakAfter
ntext::new_wordBreakBefore
ntext::new_endOfWord
ntext::new_startOfNextWord
ntext::new_startOfPreviousWord
Each function calculates the five regexp search patterns that
define the word boundary searches. These values are stored in the
namespace variables listed above.
::ntext::initializeMatchPatterns
-
This function is called when Ntext is first used, and
needs to be called again only if the script changes the value of
either ::ntext::classicWordBreak or ::tcl_platform(platform). The function is
called with no arguments. It is useful when the desired search
patterns are the default patterns for either the Ntext or
Text binding tag, and so are implicitly specified by the
values of ::ntext::classicWordBreak and ::tcl_platform(platform) alone.
::ntext::createMatchPatterns new_nonwordchars new_word1chars ?new_word2chars?
-
This function is useful in a wider range of situations than ::ntext::initializeMatchPatterns. It
calculates the regexp search patterns for any case with one class
of "non-word" characters and one or two classes of "word"
characters.
Each argument should be a regexp expression defining a class of
characters. An argument will usually be a bracket expression, but
might alternatively be a class-shorthand escape, or a single
character. The third argument may be omitted, or supplied as the
empty string, in which case it is unused.
The first argument is interpreted as the class of non-word
characters; the second argument (and the third, if present) are
classes of word characters. The classes should include all possible
characters and will normally be mutually exclusive: it is often
convenient to define one class as the negation of the other
two.
The problem of word boundary selection is a vexed one, because
text is used to represent a universe of different types of
information, and there are no simple rules that are useful for all
data types or for all purposes.
Ntext attempts to improve on the facilities available
in classic Text by providing facilities for more complex
definitions of words (with three classes of characters instead of
two).
What is a word? Why two classes of word?
When using the modified cursor keys <Control-Left> and
<Control-Right> to navigate through a Ntext widget,
the cursor is placed at the start of a word. A word is defined as a
sequence of one or more characters from only one of the two defined
"word" classes; it may be preceded by a character from the other
"word" class or from the "non-word" class.
The double-click of mouse button 1 selects a word of text, where
in this case a "word" may be as defined above, or alternatively may
be a sequence of one or more characters from the "non-word" class
of characters.
Traditionally Tcl has defined only one word class and one
non-word class: on Windows, the non-word class is whitespace, and
so alphanumerics and punctuation belong to the same class. On other
platforms, punctuation is bundled with whitespace as "non-word"
characters. In either case, the navigation and selection of text
are unnecessarily coarse-grained, and sometimes give unhelpful
results.
The use of three classes of characters might make selection too
fine-grained; but in this case, holding down the Shift key
and double-clicking another word is an excellent way to select a
longer range of text (a useful binding that Tcl/Tk has long
provided but which is missing in other systems).
As well as its defaults, Ntext permits the developer to
define their own classes of characters, or to revert to the classic
Text definitions, or to specify their own regexp matching
patterns.
To use Ntext with Tcl/Tk's usual word-boundary
detection rules:
package require ntext
text .t
bindtags .t {.t Ntext . all}
set ::ntext::classicWordBreak 1
::ntext::initializeMatchPatterns
See bindtags for more information.
To define a different set of word-boundary detection rules:
package require ntext
text .t
bindtags .t {.t Ntext . all}
::ntext::createMatchPatterns \
{[[:space:][:cntrl:]]} {[[:punct:]]} {[^[:punct:][:space:][:cntrl:]]}
See regexp, re_syntax for more information.
