Well, here's a regex function that I made for that:

  print is_quoted(q`I'm not quoted!`) . "\n" ;
  print is_quoted(q`"I'm \\"quoted!"`) . "\n" ;
  print is_quoted(q`"I'm not quoted!\"`) . "\n" ;
  print is_quoted(q`"I'm not \\\\"quoted!"`) . "\n" ;
  print is_quoted(q`""`) . "\n" ;
  print is_quoted(q`''`) . "\n" ;
  print is_quoted(q`"x"`) . "\n" ;
  print is_quoted(q`'x'`) . "\n" ;


#############
# IS_QUOTED #
#############

sub is_quoted {
  my $data = shift ;
  print "<<$data>>\n" ;
  
  $data =~ s/\\\\//gs ;   ## Ignore \\
  
  if (
  $data =~ /^             ## Begin
    \s*                   ## Ignore spaces in begin.
    (?:(?!\\).|)          ## Quote without \ before.
    (?:
      "[^"\\]?"                  ## Blank quoted or unique.
      |
      "(?:(?:\\")|[^"])+(?!\\)." ## quoted with values without " insid
+e, but accept \"
      |
      '[^'\\]?'                  ## Blank quoted or unique.
      |
      '(?:(?:\\')|[^'])+(?!\\).' ## quoted with values without ' insid
+e, but accept \'
    )
    \s*                   ## Ignore spaces in end.
  $/sx                    ## End. /x => extend space for the regex.
  ) { return( 1 ) ;} ## return true
  return 0 ;         ## return false

  ##better:
  # return undef ;
}
[download]

Note that I ignore spaces before and after the quoted values. If you don't want, just remove the \s* from the regex.

I have added the print() inside the function just to follow the tests. Here's the output:

<<I'm not quoted!>>
0
<<"I'm \"quoted!">>
1
<<"I'm not quoted!\">>
0
<<"I'm not \\"quoted!">>
0
<<"">>
1
<<''>>
1
<<"x">>
1
<<'x'>>
1
[download]

Enjoy!

Graciliano M. P.
"The creativity is the expression of the liberty".