sub timestamp2regex {
    my $exp = shift;
    my %metareplacements = (
        'D' => '%m/%d/%y',        'F' => '%Y-%m-%d',
        'r' => '%I:%M:%S %p',     'R' => '%H:%M',
        'T' => '%H:%M:%S'
    );
    my %replacements = (
        'a' => '[[:alpha:]]+',    'A' => '[[:alpha:]]+',
        'b' => '[[:alpha:]]+',    'B' => '[[:alpha:]]+',
        'd' => '\d{2}',           'e' => '[\d\s]\d',
        'g' => '\d{2}',           'G' => '\d{4}',
        'h' => '[[:alpha:]]+',    'H' => '\d{2}',
        'I' => '\d{2}',           'j' => '\d{3}',
        'k' => '[\d\s]\d',        'l' => '[\d\s]\d',
        'm' => '\d{2}',           'M' => '\d{2}',
        'p' => '[A-Za-z.]{2,}',   'P' => '[A-Za-z.]{2,}',
        's' => '\d+',             'S' => '\d{2}',
        't' => '\t',              'u' => '\d',
        'U' => '\d{2}',           'V' => '\d{2}',
        'w' => '\d',              'W' => '\d{2}',
        'y' => '\d{2}',           'Y' => '\d{4}',
        'z' => '[+-]\d{4}',       'Z' => '[[:alpha:]]*',
        '%' => '\%'
    );

    $exp = quotemeta($exp);
    $exp =~ s/\\\%\\?(.)/
        if (defined $metareplacements{$1}) {
            timestamp2regex($metareplacements{$1});
        } elsif (defined $replacements{$1}) {
            $replacements{$1};
        } else {
            croak "Unsupported or unrecognized timestamp format token: \%$1.";
        }/eg;
    return $exp;
}