List trim

dbohdan 2014-06-15: The following procedure removes items at both ends of a list that match a regexp. By default this amounts to removing empty items.

# Remove empty items at the beginning and the end of a list.
proc ltrim {list {emptyRegExp "^$"}} {
    set first [lsearch -not -regexp $list $emptyRegExp]
    set last [lsearch -not -regexp [lreverse $list] $emptyRegExp]
    return [
        if {$first == -1} {
            list
        } else {
            lrange $list $first end-$last
        }
    ]
}

Use example

eltclsh > ltrim {{} {} a b c {} 1 2 {} {} {} {} {}}
a b c {} 1 2
eltclsh > ltrim {{   } {} a b c { } 1 2 { } {} " \t " {} "\t\t"} {^[ \t]*$}
a b c { } 1 2