Validate entry with BWidget's ComboBox

The BWidget ComboBox widget allows you to fill in an entry field with a value. I would like to enforce only specific characters in that field (e.g. only [a-z0-9]). For that purpose I would like to use Tcl/Tk's -validatecommand (or -vcmd for short), just as you do with the standard 'entry' widget:

proc ValidateMyEntry { value } {
    # Check if it's alphanum string

    if ![regexp {^[-a-zA-Z0-9]*$} $value] {
        return 0
    }
    return 1
}

entry .my_entry -width 20 -textvariable myVar -validate key -vcmd {ValidateMyEntry %P}

It seems ComboBox does not support -validatecommand. What's the best work-around?

As something that was possible but a bit cumbersome, I decided to use the old-style 'trace variable' function to enforce values in combobox.

Put the following statement after the ComboBox call:

trace variable myVar w forceAlphaNum

Elsewhere, you have to define the forceAlphaNum proc:

proc forceAlphaNum { name el op } {
    if { $el == "" } {
        set newname $name
        set oldname ${name}_alphanum
    } else {
        set newname ${name}($el)
        set oldname ${name}_alphanum($el)
    }

    global $newname
    global $oldname

    if { ![info exist $oldname] } {
        set $oldname ""
    }    

    # Check if it's alphanum string
    if ![regexp {^[a-zA-Z0-9]*$} [set $newname]] {
        set $newname [set $oldname]
        bell; return
    }
    set $oldname [set $newname]
}