That's what test2.tcl in this
[http://www.itworld.com/AppDev/4061/swol-1218-regex/]
"Regular Expressions" [http://www.regularexpressions.com] column does.

Also see the  [fileevent] page.  


The sample chapter 22 from [Brent Welch]'s book can be downloaded from his home page and ( "TK Examples", or "TK by Example" ) [http://www.beedub.com/book/3rd/Tkexampl.pdf] also gives an example of this.  
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The link on top may not clarify this at a glance. I myself was thinking about opportunities with pipes and send, and how to handle remote command execution with both methods. See [Playing with Pipes, Send and DDE] - [RJM]

Reference to [text] widget.
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Can anyone point to an example where a text widget displays the [stdout] and
[stderr] from a command, as it is produced - not waiting until the
execution finishes?


[Rfox] 3/8/2012 - This is done by the NSCLDAQ Readout Gui.  The data taking program is a command driven program (Tcl Interpreter), the stdout and stderr of this program, which may be run over an ssh pipe, are captured in an output window as they occur as often experimenters will insert debugging output.
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[Category GUI]