Here's yer tuit... 
use Benchmark;

my $string = "1234567890";

timethese(-5, {
                'Regexp'      => sub { $string =~ s/(.*?)(....)$/("X" 
+x length($1)).$2/e; },
                'Regexp D&P'  => sub { $string =~ s/.(?!.{0,3}$)/x/g; 
+},
                'Substring'   => sub { $string = length substr($string
+,0,-4). substr($string,-4) }
              });

[download]
Yields: 
Benchmark: running Regexp, Regexp D&P, Substring, each for at least 5 
+CPU seconds...
    Regexp:  6 wallclock secs ( 5.38 usr +  0.00 sys =  5.38 CPU) @ 17
+3245.35/s (n=932060)
Regexp D&P:  6 wallclock secs ( 5.30 usr +  0.00 sys =  5.30 CPU) @ 20
+0480.57/s (n=1062547)
 Substring:  7 wallclock secs ( 5.45 usr +  0.00 sys =  5.45 CPU) @ 10
+04355.05/s (n=5473735)

[download]
This is on a 2 GHz Pentium 4. Let's say we increase $string to 1000 characters... 
Benchmark: running Regexp, Regexp D&P, Substring, each for at least 5 CPU seconds...
    Regexp:  6 wallclock secs ( 5.29 usr +  0.00 sys =  5.29 CPU) @ 4194.33/s (n=22188)
Regexp D&P:  5 wallclock secs ( 5.26 usr +  0.00 sys =  5.26 CPU) @ 2068.25/s (n=10879)
 Substring:  7 wallclock secs ( 5.26 usr +  0.00 sys =  5.26 CPU) @ 1022051.33/s (n=5375990)
Update: Added Dog_and_Pony's solution...
Update 2:Fixed stupid bug in Substring code. , and . do different things...
Update 3:abstracts spotted that my substring function has a cut and paste error (sheesh). jsprat's benchmark is accurate. The four arg substring seems to be the fastest.

/\/\averick
OmG! They killed tilly! You *bleep*!!

In reply to Re: s///e by maverick
in thread How do I replace all characters in a string with X's except the last 4 character by Anonymous Monk

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