s{
      (                         # $1 for whole match
         ' (?: \\. | [^']+ )* ' # 'string'
       | " (?: \\. | [^"]+ )* " # "string"
       | (?:^|\r*\n)[^\S\n]*    # leading spaces
       | ([^\S\n]+)             # $2 is trivial spaces
      )
    }{
        $2 ? ' ' : $1
    }gsex