use strict;
use warnings;

for my $reg ('(x\d{3,}?)', '(x\d{3,}?x)', '(x\d{3,})', '(x\d{3})') {
    for my $str ('xx', 'x12x', 'x123456x', 'x12x x123x') {
        print "Matched using $reg: $1\n" if $str =~ $reg;
    }
}

##</code><code>##

Matched using (x\d{3,}?): x123
Matched using (x\d{3,}?): x123
Matched using (x\d{3,}?x): x123456x
Matched using (x\d{3,}?x): x123x
Matched using (x\d{3,}): x123456
Matched using (x\d{3,}): x123
Matched using (x\d{3}): x123
Matched using (x\d{3}): x123