Script to extract selected entries from a bibtex file

I would recommend using a language with a battle-tested BibTeX library instead of reinventing that wheel. For example

#!/usr/bin/env perl
use strict;
use warnings;
use autodie;
use BibTeX::Parser;

open my $fh, '<', $ARGV[0];
my $parser = BibTeX::Parser->new($fh);
my @authoryear;
while (my $entry = $parser->next) {
    if ($entry->parse_ok) {
        if ($entry->key eq "AuthorYear") {
            push @authoryear, $entry;
        }
    }
    else {
        warn "Error parsing file: " . $entry->error;
    }
}

# I'm not familiar with bibtex files, so this may be insufficient
open my $out, '>', "authoryear.bib";
foreach my $entry (@authoryear) {
    say $out $entry->raw_bibtex;
}

You'll probably have to install the module: cpan install BibTeX::Parser

The following Python script does the desired filtering.

#!/usr/bin/python
import re

# Bibliography entries to retrieve
# Multiple pattern compilation from: http://stackoverflow.com/a/11693340/147021
pattern_strings = ['Author2010', 'Author2012',]
pattern_string = '|'.join(pattern_strings)
patterns = re.compile(pattern_string)


with open('bibliography.bib', 'r') as bib_file:
    keep_printing = False
    for line in bib_file:
        if patterns.findall(line):
            # Beginning of an entry
            keep_printing = True

        if line.strip() == '}':
            if keep_printing:
                print line
                # End of an entry -- should be the one which began earlier
                keep_printing = False

        if keep_printing:
            # The intermediate lines
            print line,

Personally, I prefer moving to a scripting language when the filtering logic becomes complex. That, perhaps, has an advantage on the readability factor at least.