Searching for IP address using regular expression

This snippet finds an IP address in a string using regex:

import re
ip = re.compile('(([2][5][0-5]\.)|([2][0-4][0-9]\.)|([0-1]?[0-9]?[0-9]\.)){3}'
                +'(([2][5][0-5])|([2][0-4][0-9])|([0-1]?[0-9]?[0-9]))')

match = ip.search("Your ip address is 192.168.0.1, have fun!")
if match:
 print 'IP address found:',
 print match.group(), # matching substring
 print 'at position',match.span() # indexes of the substring found
else:
 print 'IP address not found'

The output will be

IP address found: 192.168.0.1 at position (19, 30)

Crawling the web with SGMLParser

In this example we will use SGMLParser in order to build a simple web crawler.

import urllib
from random import choice
from sgmllib import SGMLParser

class LinkExplorer(SGMLParser): 
 def reset(self):                              
  SGMLParser.reset(self) 
  self.links = [] # list with the urls

 def start_a(self, attrs):
  """ fill the links with the links in the page """
  for k in attrs:
   if k[0] == 'href' and k[1].startswith('http'): 
    self.links.append(k[1])

def explore(parser,s_url,maxvisit=10,iter=0):
 """ pick a random link in the page s_url
     and follow its links recursively """
 if iter < maxvisit: # it will stop after maxvisit iteration
  print '(',iter,') I am in',s_url
  usock = urllib.urlopen(s_url) # download the page
  parser.reset() # reset the list
  parser.feed(usock.read()) # parse the current page
  if len(parser.links) > 0:
   explore(parser,choice(parser.links),maxvisit,iter+1)
  else: # if the page has no links to follow
   print 'the page has no links'

# test the crawler starting from the python's website
parser = LinkExplorer()
explore(parser,"http://www.python.org/")

Let's go!

( 0 ) I am in http://www.python.org/
( 1 ) I am in http://wiki.python.org/moin/NumericAndScientific
( 2 ) I am in http://numpy.scipy.org/
( 3 ) I am in http://sphinx.pocoo.org/
( 4 ) I am in http://www.bitbucket.org/birkenfeld/sphinx/issues/
( 5 ) I am in http://blog.bitbucket.org
( 6 ) I am in http://haproxy.1wt.eu/
( 7 ) I am in http://www.olivepeak.com/blog/posts/read/free-your-port-80-with-haproxy
( 8 ) I am in http://www.olivepeak.com/peaknotes/
( 9 ) I am in http://notes.olivepeak.com/account/create

Sierpinski fractal

As described in the introduction of Numerical Computing in Matlab we can generate fractals using affine transformations. The following script uses it to generate the famous Sierpinski triangle:

from numpy import *
import pylab

x = [0, 0];

A = [ [.5, 0], [0, .5] ];
b1 = [0, 0];
b2 = [.5, 0];
b3 = [.25, sqrt(3)/4];

for i in range(3000): # 3000 points will be generated
 r = fix(random.rand()*3)
 if r==0:
  x = dot(A,x)+b1
 if r==1:
  x = dot(A,x)+b2
 if r==2:
  x = dot(A,x)+b3
 pylab.plot(x[0],x[1],'m.',markersize=2)

pylab.show()

It will take a while, here's the result