First there's the solution that kvale gave, perhaps the best simplest choice: 
/^([01])[01]*$/

[download]
but there are some other possibilities using lookahead: 
/^(?=(.))[01]+$/

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

[download]

I feel a benchmark coming up... 

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

[download]
Result: 
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) @ 24
+988.64/s (n=76965)
                          Rate lookahead_then_capture capture_in_looka
+head plain
lookahead_then_capture 24592/s                     --                 
+ -1%   -2%
capture_in_lookahead   24782/s                     1%                 
+  --   -1%
plain                  24989/s                     2%                 
+  1%    --

[download]
Well... "plain" appears to be slightly faster, but I wouldn't bother worrying about the difference. Whjat's a few percent, anyway?

In reply to Re: Regex failure interpretation by bart
in thread Regex failure interpretation by BrowserUk

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