>perl -wMstrict -le
"my $pre  = qr{ \A | \s }xms;
 my $post = qr{ \s | \z }xms;
 my $str  = q{foo='123' barfoo='987' bar='555'};
 for my $user_supplied (@ARGV) {
   my $rx = qr{ $pre \Q$user_supplied\E $post }xms;
   $str =~ m{ ($rx) }xms;
   print qq{[$user_supplied] matches }, $1 ? qq{[$1]} : q{nothing};
   }
"
foo='123' bar='555' barfoo='987' foo='987'
[foo='123'] matches [foo='123' ]
[bar='555'] matches [ bar='555']
[barfoo='987'] matches [ barfoo='987' ]
[foo='987'] matches nothing