It strikes me that "similarity scoring" is the sort of gadget
that attracts/inspires RS, Arjen, ...  If I leave a mention of
[http://groups.google.com/groups?hl=en&frame=right&th=4c0bf2d2c3fb160a]
here, will they exhibit Tcl examples?

[RS] can't withstand a challenge... Indeed, I have often been wishing for such a measuring device - thanks for the link! Here's a plump translation to Tcl of the Python version of the Levenshtein algorithm given there (where it hurt to have to do all index arithmetics with [expr], so I introduced a helper subtractor), plus an application of ''stringDistance'' to compute ''stringSimilarity'', where the only little gag is that we have to determine the sum of the string lengths only once, as they're concatenated:
 proc stringDistance {a b} {
	set n [string length $a]
	set m [string length $b]
	for {set i 0} {$i<=$n} {incr i} {set c($i,0) $i}
	for {set j 0} {$j<=$m} {incr j} {set c(0,$j) $j}
	for {set i 1} {$i<=$n} {incr i} {
	   for {set j 1} {$j<=$m} {incr j} {
		set x [expr {$c([- $i 1],$j)+1}]		
		set y [expr {$c($i,[- $j 1])+1}]
		set z $c([- $i 1],[- $j 1])
		if {[string index $a [- $i 1]]!=[string index $b [- $j 1]]} {
			incr z
		}
		set c($i,$j) [min $x $y $z]
	    }
        }
	set c($n,$m)
 }
 # some little helpers:
 proc min args {lindex [lsort -real $args] 0}
 proc max args {lindex [lsort -real $args] end}
 proc - {p q} {expr {$p-$q}}

 proc stringSimilarity {a b} {
	set totalLength [string length $a$b]
	max [expr {double($totalLength-2*[stringDistance $a $b])/$totalLength}] 0.0
 }
# Testing...
 % stringSimilarity hello hello  ;# identity implies perfect similarity
 1.0
 % stringSimilarity hello hallo  ;# changed one out of five letters
 0.8
 % stringSimilarity hello Hallo  ;# case matters
 0.6
 % stringSimilarity hello world  ;# one match of five (e or o)
 0.2
 % stringSimilarity hello helplo ;# insert costs slightly less
 0.818181818182
 % stringSimilarity hello again  ;# total dissimilarity
 0.0
[[Nice work, of course; I particularly applaud the example evaluations.]]
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[Additional string functions] - [Arts and crafts of Tcl-Tk programming]