Remove extra whitespace between string parts in POSIX shell

Question

Example:

1. Input string: 'a string with "some extra space"'
2. Desired output: 'a string with "some extra space"'

How do I get from 1. to 2. without using bash-specifics? sed, awk and all standard utilities are available – even python3 if need be. Problems:

• string delimiters might be flipped and/or nested – anything between a set of arbitrary quotes should be considered a string and left untouched
• piping through tr -s ' ' removes the spaces in the last string
• xargs removes string delimiters (such as " in the example)
• echo $VAR ignores string boundaries entirely

Answer 1

Going the Python route:

#!/usr/bin/env python3
import re, shlex, sys

words = shlex.split(sys.argv[1])
print(' '.join([ f'"{word}"' if re.search(r'\s', word) else word for word in words ]))

...wrapped for use from a shell:

remove_unquoted_spaces() {
  python3 -c '
import re, shlex, sys
words = shlex.split(sys.argv[1])
print(" ".join([ f"\"{word}\"" if re.search(r"\s", word) else word for word in words ]))
' "$@"
}
 
remove_unquoted_spaces 'a    string   with    "some   extra space"'