This perl script reads Standard server access logs and converts it to CSV, the output will only contain visits from Googlebot. The verification is done using Google's recommendation by host and reverse host check and not by just pattern.

GoogleAccessLog2CSV.pl

#!/usr/bin/perl

# Original source code comes from https://github.com/woonsan/accesslog2csv
# Updated to validate GoogleBot

use Socket;
sub googlebot($)  {

    # check to see if this IP really is a Googlebot

    my $ip = shift;
    my $bot = 'googlebot\.com';
    my $name = gethostbyaddr(inet_aton($ip), AF_INET) or return 0;
    my @addr = gethostbyname($name);
    if($addr[4]) {
      my $addr = inet_ntoa($addr[4]);      
      return ($name =~ m/$bot/ and $ip eq $addr) ? 1 : 0;
    }
    return 0;
}


if ("$ARGV[0]" =~ /^-h|--help$/) {
  print "Usage: $0 access_log_file > csv_output_file.csv\n";
  print "   Or, $0 < access_log_file > csv_output_file.csv\n";
  print "   Or, $0 < access_log_file > csv_output_file.csv 2> invalid_lines.txt\n";
  exit(0);
}

%MONTHS = ( 'Jan' => '01', 'Feb' => '02', 'Mar' => '03', 'Apr' => '04', 'May' => '05', 'Jun' => '06',
  'Jul' => '07', 'Aug' => '08', 'Sep' => '09', 'Oct' => '10', 'Nov' => '11', 'Dec' => '12' );

print STDOUT "\"Id\",\"Host\",\"Log Name\",\"Date Time\",\"Time Zone\",\"Method\",\"URL\",\"Response Code\",\"Bytes Sent\",\"Referer\",\"User Agent\"\n";
$line_no = 0;

while (<>) {
  ++$line_no;
  if (/^([\w\.:-]+)\s+([\w\.:-]+)\s+([\w\.-]+)\s+\[(\d+)\/(\w+)\/(\d+):(\d+):(\d+):(\d+)\s?([\w:\+-]+)]\s+"(\w+)\s+(\S+)\s+HTTP\/1\.\d"\s+(\d+)\s+([\d-]+)((\s+"([^"]+)"\s+")?([^"]+)")?$/) {
    $host = $1;
    $other = $2;
    $logname = $3;
    $day = $4;
    $month = $MONTHS{$5};
    $year = $6;
    $hour = $7;
    $min = $8;
    $sec = $9;
    $tz = $10;
    $method = $11;
    $url = $12;
    $code = $13;
    if ($14 eq '-') {
      $bytesd = 0;
    } else {
      $bytesd = $14;
    }
    $referer = $17;
    $ua = $18;

    if(googlebot($host)) {
      print STDOUT "\"Crawl $line_no\",\"$host\",\"$logname\",\"$year-$month-$day $hour:$min:$sec\",\"GMT$tz\",\"$method\",\"$url\",$code,$bytesd,\"$referer\"\,\"$ua\"\n";
    } else {
      print STDERR "Invalid Google IP at $line_no: $_";
    }

  } else {
    print STDERR "Invalid Line at $line_no: $_";
  }
}

How to run

$ perl GoogleAccessLog2CSV.pl serverfile.log > verified_googlebot_log_file.csv

Also, you can check invalid log lines by redirecting STDERR, too:

$ perl GoogleAccessLog2CSV.pl < serverfile.log > verified_googlebot_log_file.csv 2> invalid_log_lines.txt