/^([01])[01]*$/

##</code><code>##

/^(?=(.))[01]+$/

##</code><code>##

/^(?=[01]+$)(.)/

##</code><code>##

use Benchmark 'cmpthese';
cmpthese(-3, {
   plain => sub { my $x; ($x) = /^([01])[01]*$/ foreach qw(0 1 00 11 10 01 000x00) },
   capture_in_lookahead => sub { my $x; ($x) = /^(?=(.))[01]+$/ foreach qw(0 1 00 11 10 01 000x00) },
   lookahead_then_capture => sub { my $x; ($x) = /^(?=[01]+$)(.)/ foreach qw(0 1 00 11 10 01 000x00) },
});

##</code><code>##

Benchmark: running capture_in_lookahead, lookahead_then_capture, plain, each for at least 3 CPU seconds...
capture_in_lookahead:  3 wallclock secs ( 3.41 usr +  0.00 sys =  3.41 CPU) @ 24781.52/s (n=84505)
lookahead_then_capture:  4 wallclock secs ( 3.13 usr +  0.00 sys =  3.13 CPU) @ 24592.33/s (n=76974)
     plain:  3 wallclock secs ( 3.08 usr +  0.00 sys =  3.08 CPU) @ 24988.64/s (n=76965)
                          Rate lookahead_then_capture capture_in_lookahead plain
lookahead_then_capture 24592/s                     --                  -1%   -2%
capture_in_lookahead   24782/s                     1%                   --   -1%
plain                  24989/s                     2%                   1%    --