Problem

You want to extract from a web server log file only the information you're interested in.

Solution

Pull apart the log file as follows:

while (<LOGFILE>) {
  my ($client, $identuser, $authuser, $date, $time, $tz, $method,
      $url, $protocol, $status, $bytes) =
  /^(\S+) (\S+) (\S+) \[([^:]+):(\d+:\d+:\d+) ([^\]]+) "(\S+) (.*?) (\S+)"
      (\S+) (\S+)$/;
  # ...
}

Discussion

This regular expression pulls apart entries in Common Log Format, an informal standard that most web servers adhere to. The fields are:

client

IP address or domain name of browser's machine

identuser

If IDENT (RFC 1413) was used, what it returned

authuser

If username/password authentication was used, whom they logged in as

date

Date of request (e.g., 01/Mar/1997)

time

Time of request (e.g., 12:55:36)

tz

Time zone (e.g., -0700)

method

Method of request (e.g., GET, POST, or PUT)

url

URL in request (e.g., /~user/index.html)

protocol

HTTP/1.0 or HTTP/1.1

status

Returned status (200 is okay, 500 is server error)

bytes

Number of bytes returned (could be "-" for errors, redirects, and other non-document transfers)

Other formats include the referrer and agent information. The pattern needs only minor changes for it to work with other log file formats. Watch out that spaces in the URL field are not escaped. This means that we can't use \S* to extract the URL. .* would cause the regex to match the entire string and then backtrack until it could satisfy the rest of the pattern. We use .*? and anchor the pattern to the end of the string with $ to make the regular expression engine match nothing and then add characters until the entire pattern is satisfied.

See Also

The CLF spec at http://www.w3.org/Daemon/User/Config/Logging.html