Reg Ex

In what language (JavaScript, Python, C#, etc)?

HTH,
Dan

Unless you're using specific regex tools that allow replacement string conditionals, you'll need to repeat a replacement regex until no more matches are found. If you repeat the following, it should do the job:

Pattern:

(\d)(\d{3})((?:,\d{3})*)$

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Replacement:

$1,$2$3

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Demo:
https://regex101.com/r/rfE5IO/1

This is not a good application for a regex.  It can be done per Terry's example above, but it's clunky.

Most programming languages include thousands separator support in the output / format translation routines, e.g. printf in C with the %'d formatter.  As a bonus, in linux the ' formatter is dependent on the locale and uses either full stop or comma as appropriate.

Finally, if the language offers no support at all, write your own reformatter like the one below and you won't need to worry about how many capture groups were captured, or which ones.

convert binary number to decimal characters
push characters on stack
until stack is empty
if more than three characters remain
  unstack three characters reversed to output buffer
  add comma to output buffer
else
 unstack remainder reversed to output buffer

So:  7846263
Push to stack 7, 8, 4, 6, 2, 6, 3
Unstack 3, 6, 2 -> 263
Add comma -> ,
Unstack 6, 4, 8 -> 846,263
Add comma -> ,846,263
Unstack 7 -> 7,648,263

Thanks for the replies.