$ perl -le'
my @strings = ( "a=1 gibberish b=2 c=3", "a=1 gibberish c=3" );

for ( @strings ) {
    print;
    my ( $a, $b, $c ) = / (?= .* \ba = (\S+) )? (?= .* \bb = (\S+) )? 
+(?= .* \bc = (\S+) )? /x;
    print "\$a = $a  \$b = $b  \$c = $c";
    }
'
a=1 gibberish b=2 c=3
$a = 1  $b = 2  $c = 3
a=1 gibberish c=3
$a = 1  $b =   $c = 3
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