Tcl: regsub replacing by proc

proc regsub2 {re inStr by {varName ""}} {
# Simple version of regsub but support substitution by proc:
# regsub2 $re $str $by, where $by can be {proc \1 \2...}
    set result $inStr
    set byCallStr [regsub -all {\\(\d+)} $by {$m\1}]
    set matchedVarsCount [lindex [regexp -about $re] 0]
    set matchedVarNames {all}
    for {set i 1} {$i <= $matchedVarsCount} {incr i} {
        lappend matchedVarNames "m$i"
    }
    set nsubs 0
    foreach $matchedVarNames [regexp -all -inline $re $result] {
        set byRes [eval $byCallStr]
        set result [regsub $re $result $byRes]
        incr nsubs
    }
    if {$varName eq ""} {
        return $result
    } else {
        upvar 1 $varName varName_
        set varName_ $result
        return $nsubs
    }
}