#!/usr/bin/perl
use strict;
use warnings;

my $timeformat = "*%H:%M:%S%% >"; # Example.

my %replacements = (
    '%' => '\%',
    'a' => '[[:alpha:]]+',
    'H' => '\d{2}',
    'M' => '\d{2}',
    'S' => '\d{2}'
);

$timeformat = quotemeta($timeformat);
$timeformat =~ s/\\\%\\?(.)/$replacements{$1}/eg;

print ("The regular expression is: $timeformat\n");