#!/usr/bin/perl -w
use strict;

my @all_keywords = (qw(one two three four five));
my @mandatory_keywords = (qw(two three four ));

my $RE_value = qr/
            (\w+)         # (1) a keyword
            \s* = \s*     # an equal sign with optional spaces
            (?:           # quoted keyword ...
            (             #
                [\'\"\`]  # (2) a quoting character
            )
            (             # (3) the quoted value:
                (?:       # either
                  \\\2    # an escaped quote
                  |       # or
                  [^\2]   # any non-quote character
                )
                +?        # repeat (non-greedily)
            )
            \2            # until the initial quote shows up again
            |
            (\S+)         # (4) ... bare word value
            )
            /x;

sub set_value {
    my ($stat, $kw, $value) = (@_);
    # case insensitive keyword
    return 0 unless exists $stat->{lc $kw};
    $stat->{lc $kw} = $value;
    return 1
}

sub parse_pairs {
    my $src = shift;
    my %statement = map {$_, undef} @all_keywords;
    for ($src) {
        while ( ! m/ \G \s* \z /gcx ) {
            my $result = 0;
            if ( /\G \s* $RE_value \s* /xgc ) {
                $result = set_value( \%statement, $1, $4 ? $4 : $3 );
            }
            else {
                die "syntax error >" . substr($_, pos) ."\n";
            }
            die "invalid keyword $1 \n" unless $result;
        }
    }
    return \%statement;
}

sub check_pairs {
    my $statement = shift;
    for my $kw (@all_keywords) {
        if (defined $statement->{$kw}) {
            print "$kw \t -> <$statement->{$kw}>\n"
        }
        else {
            warn "- missing keyword <$kw>!\n"
                if grep {$kw eq $_} @mandatory_keywords;
        }
    }
}

my @sources = (
  q{ ONE="xyz\t" two=a_34 three = 'name="O\'Hara"' four=`'one' two` five = ah! },
  q{
    five = ah!
    ONE="xyz\t"
    three = 'name="O\'Hara" two=a_34'
    four=`'one' two`
    });

for (@sources) {
    print "\n>>Source: //$_//\n\n";
    my $stat = parse_pairs($_);
    check_pairs($stat);
}

__END__
output:
>>Source: // ONE="xyz\t" two=a_34 three = 'name="O\'Hara"' four=`'one' two` five = ah! //

one      -> <xyz\t>
two      -> <a_34>
three    -> <name="O\'Hara">
four     -> <'one' two>
five     -> <ah!>

>>Source: //
    five = ah!
    ONE="xyz\t"
    three = 'name="O\'Hara" two=a_34'
    four=`'one' two`
    //

one      -> <xyz\t>
- missing keyword <two>!
three    -> <name="O\'Hara" two=a_34>
four     -> <'one' two>
five     -> <ah!>