Re: validating a quoted string

[Corion]

How does your regular expression fail?

A first step would be to explain the regular expression and tell us in English words what ([.]?) is supposed to do.

Ideally, describe in English what the complete regular expression should do.

My approach to solving the problem would be to take all parts of a string between double quotes and check each part that it doesn't contain a space. Alternatively, look at all "words" that are delimited with whitespace, and check that they either contain no double quotes or start with a double quote and end with a double quote.

Update: I misread part of the problem:

my "dog shepherd"     <-- valid

So that would imply that simply checking for an even number of double quotes is the simplest part.

[hellosarathy]

Sorry, My bad, typo, it should be

$in =~ /(["]?)[^" ]+\1/

[hellosarathy]

my query:

 
$in =~ /(["]?)[^" ]+\1/

does'nt work as expected. I can have a string separated by spaces. Sometimes I can also have two or more strings within quotes to be considered as a single string (just like command line arguments we pass to a script).

[Corion]

A problem with your current regular expression is that [^" ]+ does not allow spaces.

This fails for example for the following string, which I think should be valid:

Corion says "hello sarathy"

If you want to stay with your approach of matching the tokens (words or quoted parts), I suggest reading perlre, especially on alternation.

For such an approach, I would restate the problem as Match every token that starts with a letter and consists only of letters, or starts with a double-quote and consists of non-double-quotes..

[BillKSmith]

I think that you want to parse the string the same way that a shell parses a command-line. The result is a list of sub-strings. It probably is a good idea to do the validation (use previous suggestions) before attempting the parse. I cannot think of a good way to do the parsing. I hope I have put other monks on the right track.

UPDATE

You can parse the strings with Text::CSV

#!/usr/bin/perl
use warnings;
use strict;
use Text::CSV;

my @strings = (
    '"my" "dog"',
    '"my" "dog shepherd"',
    'my dog',
    'my "dog shepherd"',
    '"my "dog"',
    'my "dog shepherd',
);
my $csv = Text::CSV->new ( {sep_char => ' '} )
    or die "Cannot use CSV: ".Text::CSV->error_diag ();

foreach my $string (@strings){
    open my $fh, '<', \$string or die "Cannot open string";
    if (((my $temp = $string) =~ tr/"//) % 2){
        warn "invalid string";
        next;
    }
    my $row = $csv->getline($fh);
    if (!defined $row) {
        warn "getline failed";
        next;
    }
    close $fh;
    $" = ' | ';
    print "@$row\n";
}