AMG: Neat! Here, this proc does the reverse (adds underscores):

proc abnorm {num} {
    if {![string is double -strict $num]} {
        error "not a number: \"$num\""
    }
    set point [string first . $num]
    if {$point == -1} {
        set point [string length $num]
    } else {
        for {set pos [expr {$point + 4}]} {$pos < [string length $num]} {incr pos 4} {
            set num [string replace $num $pos $pos _[string index $num $pos]]
        }
    }
    for {set pos [expr {$point - 3}]} {$pos > 0} {incr pos -3} {
        set num [string replace $num $pos $pos _[string index $num $pos]]
    }
    return $num
}

Example:

% abnorm 100.10100
100.101_00
% abnorm 10000000
10_000_000
% abnorm 10000100.101
10_000_100.101

This code doesn't handle exponential notation or even negative numbers. I don't want to put much more effort into it, since I'm more interested in seeing some clever [regsub] version. If anyone wants to give it a try, ... go right ahead. Have a blast.

AMG: I woke this morning with an inspiration about how to do this with [regsub].

proc sreverse {str} {
    set result ""
    for {set i [expr {[string length $str] - 1}]} {$i >= 0} {incr i -1} {
        append result [string index $str $i]
    }
    return $result
}

proc abnorm {num} {
    if {![string is double -strict $num]} {
        error "not a number: \"$num\""
    }
    regexp {^(-|\+)?(\d+)?(\.)?(\d+)?(e|E)?(-|\+)?(\d+)?$} $num num sm int dot frac e se exp
    set result $sm
    append result [regsub -all {(?!^)(?=(\d{3})+$)} $int _]
    append result $dot
    append result [sreverse [regsub -all {(?!^)(?=(\d{3})+$)} [sreverse $frac] _]]
    append result $e$se$exp
    return $result
}

I don't bother putting _'s in the exponent because we can't have exponents 1000 and up. I have to do the [sreverse] bit because while we may have look-ahead regular expressions, we do not have look-behind.

AMG 2006-12-15: There is now a [string reverse] command.

RS proposes this simpler version of sreverse:

proc sreverse str {
    set res ""
    set i [string length $str]
    while {$i} {append res [string index $str [incr i -1]]}
    set res
}
% sreverse hello
olleh

AMG: I saw that on the Additional string functions page, but I chose not to use it because even though it may be more efficient there's a few things I don't like about its style. (This doesn't reflect on your code, merely my preferences.)

• I feel that it's an abuse to consider 0 to be false and nonzero to be true. I always explicitly make the comparison with zero. Java requires this, by the way. I find this especially important when using [string compare], where a naive reader would assume that ![string compare] tests for string inequality.
• In many cases I dislike simultaneously using a command's side effect and its return value. Does [incr i -1] return the original value of $i or the changed value? I know the answer, but I would like it to be evident from the code structure.
• [set res] bothers me because I'm accustomed to seeing [return]. I like being explicit about the fact that a proc returns a value, rather than relying on the (standard, documented) fall-off-the-bottom behavior.
• I always brace the second argument to [proc] because that reduces the chance of typo if I add arguments or defaults later on.

But I also see that my code's use of [expr] makes it hard to read. [for] is always funny, too. If not for the high price, I might consider using the [unknown] I mention at the bottom of the page of the same name, and write:

proc sreverse {str} {
    set result ""
    foreach i [[expr {[string length $str] - 1}]-0] {
        append result [string index $str $i]
    }
    return $result
}

Hmm, that's pretty weird-lookin' too. Now you've got me wishing for $(...) syntax for [expr], but we can never have that because it's already used for arrays named "". $[...] maybe? Ah, I know how to do it without (explicit) [expr]. This is quite nice, I think:

proc sreverse {str} {
    set result ""
    for {set i 0} {$i < [string length $str]} {incr i} {
        append result [string index $str end-$i]
    }
    return $result
}

Oh, look what we've done. We hijacked SYStems's page and turned it into a style discussion! - RS keeps on the hijacking with a collecting wrapper for simple loops:

proc loop {_var from to body} {
    upvar 1 $_var var
    set res {}
    for {set var $from} {$var < $to} {incr var} {lappend res [uplevel 1 $body]}
    set res
}
% loop i 0 10 {set i}
0 1 2 3 4 5 6 7 8 9

Then we can write:

proc sreverse {str} {
    join [loop i 0 [string length $str] {string index $str end-$i}] ""
}

% sreverse hello
olleh

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