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ntextWordBreak(n) 1.0 tklib "ntext Word Boundary Detection for the Text Widget"


ntextWordBreak - ntext Word Boundary Detection for the Text Widget

Table Of Contents


  • package require Tcl 8.5
  • package require Tk 8.5
  • package require ntext ?1.0?


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:


Advanced Use

Variables (Advanced Use)






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.

Functions (Advanced Use)

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:







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::createMatchPatterns new_nonwordchars new_word1chars ?new_word2chars?


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.


This version of ntext is intended to be compatible with all releases of Tk 8.5 and 8.6, and with the branches core-8-5-branch, core-8-6-branch, and trunk in the source code repository for Tk. Any incompatibility with any of these versions, for any Tk windowing system, should be reported as a bug. Please report such in the category ntext of the Tklib Trackers.


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

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.

See Also

bindtags, ntext, re_syntax, regexp, text


bindtags, re_syntax, regexp, text