http://purl.org/tcl/home/man/tcl8.4/TkCmd/entry.htm
An entry box is designed to allow the user to enter one line of text for use by the application.
Hopefully, people will add new pages demonstrating various techniques for doing entry box validation of some common formats, like phone numbers, etc.
Validating Credit Card Check Digits
Entry box validation for IP address
One common question is about "Echo-free password entry". Entry has an easy answer: its -show option is useful in precisely this situation.
Tk syntax help - Arts and crafts of Tcl-Tk programming - Category Command