echo "$NUM" | sed -r ':a;s/^([-+]?[0-9]+)([0-9]{3})/\1,\2/;ta'

##</code><code>##

# This function inserts commas into numbers at every 3 digits
# and returns the result in a global variable called $STR.
#
function Commify
{
  STR="$1"
  if [[ "$STR" =~ ([\+\-\$\(0-9\.]+) ]]; then
    local NEG=''
    local NUM="${BASH_REMATCH[1]}"
    local PREFIX="${STR%$NUM*}"
    local SUFFIX="${STR##*$NUM}"
    if [[ "$NUM" == *"+"* ]]; then NEG="+"; fi
    if [[ "$NUM" == *"-"* ]]; then NEG="-"; fi
    if [[ "$NUM" =~ 0+([1-9]+[0-9.]*) ]]; then NUM=${BASH_REMATCH[1]}; fi
    NUM="$NEG$NUM"
    NUM=$(echo "$NUM" | sed -r ':a;s/^([-+]?[0-9]+)([0-9]{3})/\1,\2/;ta')
    STR="$PREFIX$NUM$SUFFIX"
  fi
}