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?