use v5.12; my @should_match = ( q[foo], q[WXYZfoo], q[WXYZWXYZfoo], q[WXYZ WXYZfoo], ); my @should_not_match = ( q[ foo], q[ABCfoo], q[VWXYZfoo], q[WXYZWXYZWXYZAfoo], ); my $regexp = qr{^(?:.{4})*foo}; say /$regexp/ ? "ok" : "not ok" for @should_match; say /$regexp/ ? "not ok" : "ok" for @should_not_match;