A function to substitute parts of a string matching a regular expression with the result of a script called using the match as argument. Code and examples will be much more clear then my english:

    proc regmap {re string script} {
        set submatches [lindex [regexp -about $re] 0]
        lappend varlist idx
        while {[incr submatches -1] >= 0} {
            lappend varlist _
        }
        set res $string
        set indices [regexp -all -inline -indices $re $string]
        set delta 0
        foreach $varlist $indices {
            foreach {start end} $idx break
            set substr [string range $string $start $end]
            set toeval [string map [list %s $substr] $script]
            set subresult [eval $toeval]
            incr start $delta
            incr end $delta
            set res [string replace $res $start $end $subresult]
            incr delta [expr {[string length $subresult]-[string length $substr]}]
        }
        return $res
    }

Example:

 puts [regmap {[0-9]+} "1 20 hello 30 how are you 40" {expr %s+1}]
 puts [regmap {[a-z]+} "My name is merz" {format "%s([string length %s])"}]

The output is the following:

-- [SS]

 2 21 hello 31 how are you 41
 My(1) name(4) is(2) merz(4)