#!/usr/bin/perl -w

use Benchmark qw(cmpthese);
$s = "\bthis is an\b correct\b\b\b usage\b";

sub uk {
  $a = $s;
  $a =~ s[(?:[^\cH]\cH|^\cH)][]g while 1+index $a, chr(8);
  $a;
}

sub new1 {
  $a = $s;
  while ($a =~ s/(?:[^\cH]\cH|^\cH+)//g) {}
  $a;
}

sub smack {
  $a = $s;
  $a =~ s/^\cH+//;
  1 while ($a =~ s/[^\cH]\cH//g);
  $a;
}

cmpthese(-5,{uk=>\&uk,new1=>\&new1,smack=>\&smack});
$s x= 100;
cmpthese(-5,{uk=>\&uk,new1=>\&new1,smack=>\&smack});

Benchmark: running new1, smack, uk, each for at least 5 CPU seconds...
      new1:  6 wallclock secs ( 5.39 usr +  0.03 sys =  5.42 CPU) @ 11344.28/s (n=61486)
     smack:  6 wallclock secs ( 5.24 usr +  0.02 sys =  5.26 CPU) @ 36814.64/s (n=193645)
        uk:  4 wallclock secs ( 5.26 usr +  0.06 sys =  5.32 CPU) @ 13261.09/s (n=70549)
         Rate  new1    uk smack
new1  11344/s    --  -14%  -69%
uk    13261/s   17%    --  -64%
smack 36815/s  225%  178%    --
Benchmark: running new1, smack, uk, each for at least 5 CPU seconds...
      new1:  5 wallclock secs ( 5.22 usr +  0.05 sys =  5.27 CPU) @ 140.04/s (n=738)
     smack:  6 wallclock secs ( 5.26 usr +  0.07 sys =  5.33 CPU) @ 873.55/s (n=4656)
        uk:  5 wallclock secs ( 5.18 usr +  0.04 sys =  5.22 CPU) @ 176.44/s (n=921)
       Rate  new1    uk smack
new1  140/s    --  -21%  -84%
uk    176/s   26%    --  -80%
smack 874/s  524%  395%    --