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in thread Removing commas and dollar signs from a variable.

tr/// is faster, but s/// can be used for more complex matches.
I'll support my answer with a Benchmarked example:

#!/usr/bin/perl -w
use strict;
use Benchmark;

# Generate 1 MB of random data

my $data;
for (my $i = 0; $i <= 1048576; $i++){
    $data .= chr int rand 256;
}
my $copy = $data;

study $data;
study $copy;

# Remove X'es

Benchmark::cmpthese(-10, {
    's///'  => sub { (my $dummy = $data) =~ s/X//g; },
    'tr///' => sub { (my $dummy = $copy) =~ tr/X//d; }
});

print $copy eq $data ? "OK\n" : "NOT OK\n";

[download]

This script's output: 
Benchmark: running s///, tr///, each for at least 10 CPU seconds...
      s///: 12 wallclock secs (10.80 usr +  0.02 sys = 10.82 CPU) @ 37
+.89/s (n=410)
     tr///: 15 wallclock secs (10.08 usr +  0.08 sys = 10.16 CPU) @ 42
+.81/s (n=435)
        Rate  s/// tr///
s///  37.9/s    --  -11%
tr/// 42.8/s   13%    --
OK

[download]

Update 200112212151: I forgot to copy the string before removing the X'es. After the first iteration, there'd be no X'es left. For your entertainment, I present the previous version of my post:
Hi,

I was going to answer that tr/// was faster, and s/// can be used for more complex matches.
I was going to support my answer with a Benchmarked example, which would show that tr/// was a lot faster.

BUT my benchmark told me s/// is the winner. On 1 MB of random data, s/X//g is faster than tr/X//d. If anyone can tell me why this is, or what I'm doing wrong, I'd really appreciate that.

#!/usr/bin/perl -w
use strict;
use Benchmark;

# Generate 1 MB of random data

my $data;
for (my $i = 0; $i < 1048576; $i++){
    $data .= chr int rand 256;
}
my $copy = $data;

# Remove X'es

Benchmark::cmpthese(-10, {
    's///'  => sub { $data =~ s/X//g; },
    'tr///' => sub { $copy =~ tr/X//d; }
});

print $copy eq $data ? "OK\n" : "NOT OK\n";

[download]
This script's output: 
Benchmark: running s///, tr///, each for at least 10 CPU seconds...
      s///: 11 wallclock secs (10.52 usr +  0.01 sys = 10.53 CPU) @ 72
+.08/s (n=759)
     tr///: 10 wallclock secs (10.53 usr +  0.01 sys = 10.54 CPU) @ 61
+.29/s (n=646)
        Rate tr///  s///
tr/// 61.3/s    --  -15%
s///  72.1/s   18%    --
OK

[download]
 

2;0 juerd@ouranos:~$ perl -e'undef christmas'
Segmentation fault
2;139 juerd@ouranos:~$

[download]

In Section Seekers of Perl Wisdom