Fixing escape format when values are empty

[pauline-galea]

I am using the following format to use colour when displaying text.
an is the attribute number, bn is the background number, fn is the
foreground number.

  fmt="\e[${an};${bn};${fn}m"
  printf "${fmt}%s\e[0m" "Jane"

But "an", "bn", "fn" can be empty, and I then have to
fix fmt.

Currently I am using the following replacements, but unsure
if this solves all problems.  Or perhaps there  are simpler
way to modify fmt.

  fmt="\e[${an};${bn};${fn}m"
  fmt=${fmt/";;"/";"}   # replace [;  with [
  fmt=${fmt/"[;"/"["}   # replace [;; with [