In my benchmarks, substr is about twice as fast as a /..../g regex for getting the next X characters: 

cail:~/work/perl/monks$ cat 966488.pl 
#!/usr/bin/env perl
use Modern::Perl;
use Benchmark qw(:all);

my $string = '';
for (1..1_000_000){  # make a million-char string
    $string .= qw(A C G T)[rand(4)];
}
cmpthese( 100, {
    'regex'     => \&regex,
    'substring' => \&substring,
});

sub substring {
    my $str = $string;
    my %h;
    while(length($str) % 3){    # snip to 3-letter boundary
        substr($str,-1, 1, '');
    }
    while($_ = substr($str,0,3,'')){
        $h{$_}++;
    }
}

sub regex {
    my $str = $string;
    my %h;
    for ($str =~ /.../g){
        $h{$_}++;
    }
}
cail:~/work/perl/monks$ perl 966488.pl 
            Rate     regex substring
regex     5.78/s        --      -49%
substring 11.4/s       97%        --

[download]

Of course, if you only want to match certain letters, then you're back to a regex. But in that case, I might still try stripping out all the stuff I don't want with tr//, followed by substr to break it into pieces.

Aaron B.
My Woefully Neglected Blog, where I occasionally mention Perl.

In reply to Re^2: How can I get correct result in counting 3-letter words? by aaron_baugher
in thread How can I get correct result in counting 3-letter words? by supriyoch_2008

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.