#! /usr/bin/perl -w
# commify.pl: commify *all* of the numbers in a file (or stdin).
# Well, almost all. Add commas to strings of at least 5 digits, that
# don't follow a decimal pt, comma, or word char, and don't start with
# a zero.
while (<>) {
  s/(?<! [.,\w]) ([1-9] \d{4,})/commify_digits($1)/egx;
  print;
}
sub commify_digits {
  local($_)= shift;
  s/(\d)(?=(\d{3})+$)/$1,/g;
  return $_;
}

=pod

A few notes:

Not commifying 4-digit numbers reduces the occurrence of unwanted
commas.  For example, we don't want "Dec 31, 2,019".  A comma doesn't
really add to the readability of a small number anyway.  A string of
digits beginning with a zero might be an octal number.  Even if it
isn't, 004,200,000 looks wrong, and 00,245 is confusing.  If there is
a word char before the digits, it might be part of a name of some
sort: var1234567, or file_424242.txt.  Or, it might be hex: 0x123456.
We don't examine the character that follows the digits, since it may
start a suffix: 1459798sec becomes 1,459,798sec.

This is far from perfect, but seems to be useful.  You can use it for
data or log files which contain large numbers.  You can also try to
commify the output of ls, du, df, etc.  For example, here are three
views of a large iso image:

  $ ls -l devuan_dvd.iso
  -rw-rw-r-- 1 jeff jeff 4658798592 Oct  5  2018 devuan_dvd.iso

  $ ls -lh devuan_dvd.iso
  -rw-rw-r-- 1 jeff jeff 4.4G Oct  5  2018 devuan_dvd.iso

  $ ls -l devuan_dvd.iso | commify
  -rw-rw-r-- 1 jeff jeff 4,658,798,592 Oct  5  2018 devuan_dvd.iso

The longer output of 'ls -l|commify' won't be as nice, since the
columns become misaligned.  It's not too bad, though, considering that
it is produced by just 7 lines of code,


The first s/// uses a negative-lookbehind to search for strings of
digits, and then passes them to commify_digits.  You can modify
'[.,\w]' to adjust which digit strings are commified, or change
'\d{4,}' to set the minimum length.  The s/// in commify_digits is
simple, since it gets an entire string of digits.  It just adds a
comma after each digit that is followed by a multiple of three (to the
end of the string).

Jeff Norden, Dec 2019.  This code is in the public domain.

=cut