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Select or download this
```
/^([01])[01]*$/
```
Select or download this
```
/^(?=(.))[01]+$/
```
Select or download this
```
/^(?=[01]+$)(.)/
```

use Benchmark 'cmpthese';
cmpthese(-3, {
...
   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) },
});

Select or download this

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 24592/s                     --                 
+ -1%   -2%
capture_in_lookahead   24782/s                     1%                 
+  --   -1%
plain                  24989/s                     2%                 
+  1%    --