On this day, I'm thankful for PerlMonks. My brethren in the monastery have been very helpful to me this year and I'm grateful for their generosity, kindness and alacrity.

I have a Perl style question about parsing using a regular expression pattern. I'm matching mutually exclusive alternatives and then testing which of the four alternatives matched using the defined function and nested conditional statements.

use strict;
use warnings;
use English qw( -no_match_vars );

my $TOKEN_PATTERN = qr{
    ([^\\])          # 1 Literal character (g)
    |
    \\u([0-9a-f]{4}) # 2 Universal Character Name (\u263a)
    |
    \\(["^\\])       # 3 Literal character escape sequence (\")
    |
    \\([tnfr])       # 4 Control code escape sequence (\n)
}x;

my %CONTROL_CODE = (
    t => 0x09,
    n => 0x0a,
    f => 0x0c,
    r => 0x0d,
);

while (my $line = <>) {
    chomp $line;

    while ($line =~ m/$TOKEN_PATTERN/g) {
        my $token = $LAST_PAREN_MATCH;

        # Decode tokens...
        my $code = defined $1 ? ord $token
                 : defined $2 ? hex $token
                 : defined $3 ? ord $token
                 : defined $4 ? $CONTROL_CODE{$token}
                 :              undef
                 ;

        printf "U+%04x\n", $code if defined $code;
    }
}

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Is there a better way to do this? What I'm doing works, but it feels clunky. Any suggestions for improvement?

Happy Thanksgiving!

In reply to Regex Parsing Style by Jim

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