Quoted string parsing in bash

bash_parser.sh

#!/bin/bash
set -euo pipefail

input="$(cat <&0)"

if [[ "${COMPARE_EVAL:-}" == "1" ]] ; then
  printf 'parsing of args with native bash eval:\n\n'
  eval "array=($input)"
  for token in "${array[@]}"; do
    printf '[%q]\n' "$token"
  done
  echo
fi

result=()

tokenize_shell_string() {
  local input="$1"
  local token=''
  local escape=''
  local quote=''

  # Process the input character by character
  for (( i=0 ; i<=${#input} ; i++ )) ; do
    c=${input:i:1}
    # Handle an escaped character
    if [[ -n "$escape" ]] ; then
      token+="$c"
      escape=''
    # Handles an unescaped backslash, denoting that the next char is escaped
    # Note that single quotes don't support escaping internally
    elif [[ "$c" == \\ ]] && [[ "$quote" != "'" ]] ; then
      escape="$c"
    # Handle open quotes
    elif [[ "$c" =~ [\"|\'] ]] && [[ -z "$quote" ]]; then
      quote="$c"
    # Handle matching closed quotes
    elif [[ -n "$quote" ]] && [[ "$c" == "$quote" ]]; then
      quote=''
    # Handle whitespace delimiters when outside of quotes
    elif [[ -z "$quote" ]] && [[ "$c" =~ [[:space:]] ]] ; then
      result+=("$token")
      token=''
    # End of input
    elif [[ $i == "${#input}" ]] && [[ -n "$token" ]] ; then
      result+=("$token")
      token=''
    # Any other token
    else
      # Detect unescaped dollar signs in double quotes
      if [[ "$c" == '$' ]] && [[ "$quote" == '"' ]] ; then
        return 100
      # Detect shell characters when not quoted
      elif [[ -z "$quote" ]] && [[ "$c" =~ [][\!\#\$\&\(\)\*\;\<\>\?\\\^\`\{\}] ]] ; then
        return 100
      fi
      token+="$c"
    fi
  done
}

if ! tokenize_shell_string "$input" ; then
  echo "NEEDS SHELL"
  exit 1
fi

for str in "${result[@]}" ; do
  printf "[%q]\\n" "$str"
done

invoke.bash

printf "alpaca \\'blah 'llamas\\n\\nalpa'\"'\"'cas'" | COMPARE_EVAL=1 ./bash_parser.sh