envsubst like substitution; only replacing $VAR or ${VAR} if they have a value in in env

env-susbst.sh

#!/bin/bash
# envsubst like substitution; only replacing $VAR or ${VAR} if they exists
# based on http://mywiki.wooledge.org/TemplateFiles
while read -r; do
  while [[ $REPLY =~ \$(([a-zA-Z_][a-zA-Z_0-9]*)|\{([a-zA-Z_][a-zA-Z_0-9]*)\})(.*) ]]; do
      if [[ -n ${!BASH_REMATCH[2]} ]]; then
        printf %s "${REPLY%"$BASH_REMATCH"}${!BASH_REMATCH[2]}"
      else
          printf %s "${REPLY%"$BASH_REMATCH"}\$${BASH_REMATCH[2]}"
      fi
    else # found ${var}
      if [[ -n ${!BASH_REMATCH[3]} ]]; then
        printf %s "${REPLY%"$BASH_REMATCH"}${!BASH_REMATCH[3]}"
      else
        printf %s "${REPLY%"$BASH_REMATCH"}\${${BASH_REMATCH[3]}}"
      fi
    fi
    REPLY=${BASH_REMATCH[4]}
  done
  printf "%s\n" "$REPLY"
done