[EKB] I wanted the feature (in many popular programmer's editors) of maintaining the indentation when I hit the "Enter" (or "Return") key to go to a new line. Here's what I came up with:

    proc handleEnterKey {w} {
        # First, delete the current selection
        if {[$w tag nextrange sel 1.0] != ""} {
            $w delete sel.first sel.last
        }
        
        # Find the whitespace at the start of the current line
        set startl [$w index "insert linestart"]
        set endl [lindex [split $startl "."] 0].end
        regexp {^([ \t]*)} [$w get $startl $endl] -> ws
        
        # Create a newline, insert whitespace
        $w insert [$w index insert] "\n$ws"
        # If necessary, scroll the view so cursor is visible
        $w see [$w index insert]
    }

    text .some.text.widget

    bind .some.text.widget <Return> {
        handleEnterKey %W
        # break, since we've already handled the Return
        break
    }

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See also [auto-indent]