The full expression used is '$‘\n’', which gets bash to insert a literal linefeed into the sed script. The first backslash is required by sed. The first single-quote comes out of the quoted sed code. The $‘\n’ is the bash linefeed. The last quote goes back into the quoted code.
I did this because sed’s loop markers don’t like semicolons after them, for some reason. I could of course just have typed linefeeds at those points. 
Interesting stuff!
Late as usual…
Here’s a simple one you can use in an ASObjC-based library under Mavericks:
use framework "Foundation"
on formatNumber:theNumber
set theFormatter to current application's NSNumberFormatter's new()
theFormatter's setNumberStyle:(current application's NSNumberFormatterDecimalStyle)
set theResult to theFormatter's stringFromNumber:theNumber
return theResult as text
end formatNumber:
This is faster than the sed version, but slower than MacUserII’s. However, that version assumes you’re starting of with a string like “1234567890123456789”, and my guess is that the OP is starting with a number – so it really needs to be prepared to turn 1.23456789012346E+18 into a usable string.
The other advantage it has is that it will also make Yvan happy – it will use the thousands separator the user has specified on their computer.
And for readers not sure of what that involves, see macscripter.net/viewtopic.php?pid=168044#p168044
Hello.
I’m even later, I just wrote one to return the thousand separator only, while I figure out how to get the list separator, without going into the locale with a do shell script.
framework "Foundation"
on thouSep()
tell current application's NSNumberFormatter's new()'s thousandSeparator()
return it as string
end tell
end thouSep
thouDelimit("123456789012345678901234567", ",")
on thouDelimit(n, delim)
set s to ""
repeat until (count of n) < 4
set s to delim & (text -3 thru -1 of n) & s
set n to text 1 thru -4 of n
end repeat
return n & s
end thouDelimit
edit: I overlooked Nigel’s post. It seems that his and mine are almost the same only I wrote it down in fewer lines of code.