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Chapter 15. Regular Expressions > Regular Expression Adventures

15.4. Regular Expression Adventures

We will now run through an in-depth example of the different ways of using regular expressions for string manipulation. The first step is to come up with some code that actually generates some random (but-not-so-random) data on which to operate. In Example 15.2, we present, a script which generates a data set. Although this program simply displays the generated set of strings to standard output, this output may very well be redirected to a test file.

CORE NOTE:32-bit Dates and “The End of Time”

Unix systems, as well as others, use architecture-size integers to represent the current time in seconds. Since most systems today are 32-bit, the total amount of time recognized by any platform using this mechanism is 232 seconds. Such integers are signed, so we really only have 231-1 seconds.

The current time is recognized as the number of seconds which have elapsed since time zero, which is pegged at midnight, January 1, 1970. Moving forward to the maximum possible positive 32-bit signed integer (231 - 1), we arrive at the “end of time,” which evaluates to Tuesday morning, January 19, 2038 at 3:14 AM and 7 seconds using Universal Coordinated Time (UTC/GMT). Hopefully by then, we would have discontinued the use of 32-bit systems. This phenomena is otherwise known as the Y2038 problem.)

Here is one way you could find out what the special date/time it is for your local time, using Python:

>>> import sys, time
>>> time.asctime(time.localtime(sys.maxint))# Pacific Time
'Mon Jan 18 19:14:07 2038'

sys.maxinthas the last possible second using a 32-bit integer. We feed that time in seconds to time.localtime()to obtain the tuple for your/our local time (here we are on Pacific Time), and finally, we ship that tuple off to time.asctime()to obtain the standard timestamp for the last possible second. As you can see from our example, we are eight hours west of the Prime/Greenwich Meridian.

This is not as much a Python Core Note as it is a general programming note, but should be nevertheless discussed for common knowledge since it applies to all 32-bit systems with applications using on the C language, regardless of platform, i.e., UNIX and non-UNIX, which use UNIX-style dating. In the script coming up, we randomly generate integers, effectively generating random dates for our application.


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