Run a procedure on a submatch string in a regex

proc OptionalUnicode {x} {
    return TEST$x
}

set teststring {
    this
    a[:optional_unicode(foo):]b
    c[:optional_unicode(bar):]d
    that
}

set pattern {\[\:optional_unicode\(([^)]*)\):\]}

set i 0
set output {}
while {[regexp -indices -start $i $pattern $teststring whole part]} {
    foreach {i0 i3} $whole break
    foreach {i1 i2} $part break
    append output \
	[string range $teststring $i [expr ($i0-1)]] \
	[OptionalUnicode [string range $teststring $i1 $i2]]
    set i [expr {$i3 + 1}]
}
append output [string range $teststring $i end]

puts $output