Programming‎ > ‎Python‎ > ‎

11-Regex



Regular Expressions


In computing, a regular expression, also referred to as “regex” or “regexp”, provides a concise and flexible
means for matching strings of text, such as particular characters, words, or patterns of characters. A regular
expression is written in a formal language that can be interpreted by a regular expression processor.

Really clever “wild card” expressions for matching and parsing strings

• Regular expressions are a cryptic but powerful language for matching strings and extracting elements from those strings
• Regular expressions have special characters that indicate intent

Understanding Regular Expressions


• Very powerful and quite cryptic
• Fun once you understand them
• Regular expressions are a language unto themselves
• A language of “marker characters” - programming with characters
• It is kind of an “old school” language - compact


Regular Expression Quick Guide

^         Matches the beginning of a line
$          Matches the end of the line
.           Matches any character
\s         Matches whitespace
\S         Matches any non-whitespace character
*          Repeats a character zero or more times
*?         Repeats a character zero or more times (non-greedy)
+          Repeats a character one or more times
+?         Repeats a character one or more times (non-greedy)
[aeiou]   Matches a single character in the listed set
[^XYZ]   Matches a single character not in the listed set
[a-z0-9]  The set of characters can include a range
(            Indicates where string extraction is to start
)            Indicates where string extraction is to end


The Regular Expression Module


• Before you can use regular expressions in your program, you must import the library using "import re"
• You can use re.search() to see if a string matches a regular expression, similar to using the find() method for strings
• You can use re.findall() extract portions of a string that match your
regular expression similar to a combination of find() and slicing: var[5:10]


Using re.search() like find()


hand = open('mbox-short.txt')
for line in hand:
    line = line.rstrip()
    if line.find('From:') >= 0:
        print line

import re
hand = open('mbox-short.txt')
for line in hand:
    line = line.rstrip()
    if re.search('From:', line) :
        print line


Using re.search() like startswith()


import re
hand = open('mbox-short.txt')
for line in hand:
    line = line.rstrip()
    if re.search('^From:', line) :
        print line

hand = open('mbox-short.txt')
for line in hand:
    line = line.rstrip()
    if line.startswith('From:') :
        print line

We fine-tune what is matched by adding special characters to the string


Wild-Card Characters


• The dot character matches any character
• If you add the asterisk character, the character is “any number of times”

X-Sieve: CMU Sieve 2.3
X-DSPAM-Result: Innocent
X-DSPAM-Confidence: 0.8475
X-Content-Type-Message-Body: text/plain 

^X.*:


Fine-Tuning Your Match


• Depending on how “clean” your data is and the purpose of your application, you may want to narrow your match down a bit

X-Sieve: CMU Sieve 2.3
X-DSPAM-Result: Innocent
X-Plane is behind schedule: two weeks 

^X-\S+:
^ Match the start of the line
\S Match any non-whitespace character
One or more times


Matching and Extracting Data


• The re.search() returns a True/False depending on whether the string matches the regular expression
• If we actually want the matching strings to be extracted, we use re.findall()

>>> import re
>>> x = 'My 2 favorite numbers are 19 and 42'
>>> y = re.findall('[0-9]+',x)
>>> print y
['2', '19', '42']

[0-9]+   One or more digits

• When we use re.findall(), it returns a list of zero or more sub-strings that match the regular expression

>>> y = re.findall('[AEIOU]+',x)
>>> print y
[]


Warning: Greedy Matching


• The repeat characters (* and +) push outward in both directions (greedy) to match the largest possible string

>>> import re
>>> x = 'From: Using the : character'
>>> y = re.findall('^F.+:', x)
>>> print y
['From: Using the :']

Why not 'From:' ?

^F.+:
+ One or more characters
Last character in the match is a :


Non-Greedy Matching


• Not all regular expression repeat codes are greedy! If you add a ? character, the + and * chill out a bit...
>>> import re
>>> x = 'From: Using the : character'
>>> y = re.findall('^F.+?:', x)
>>> print y
['From:']

F.+?:
+? One or more characters but not greedy
Last character in the match is a :


Fine-Tuning String Extraction


• You can refine the match for re.findall() and separately determine which portion of the match is to be extracted by using parentheses

From stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008
>>> y = re.findall('\S+@\S+',x)
>>> print y
['stephen.marquard@uct.ac.za']


• Parentheses are not part of the match - but they tell where to start and stop what string to extract

From stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008
>>> y = re.findall('\S+@\S+',x) ^From:(\S+@\S+)
>>> print y
['stephen.marquard@uct.ac.za']

>>> y = re.findall('^From:.*? (\S+@\S+)',x)
>>> print y
['stephen.marquard@uct.ac.za']


The Regex Version


From stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008

import re
lin = 'From stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008'
y = re.findall('@([^ ]*)',lin)
print y
['uct.ac.za']

'@([^ ]*)'
Look through the string until you find an at sign
Match non-blank character Match many of them


Even Cooler Regex Version


From stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008

import re
lin = 'From stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008'
y = re.findall('^From .*@([^ ]*)',lin)
print y
['uct.ac.za']

'^From .*@([^ ]*)'
Starting at the beginning of the line, look for the string 'From '
Skip a bunch of characters, looking for an at sign


Spam Confidence


import re
hand = open('mbox-short.txt')
numlist = list()
for line in hand:
    line = line.rstrip()
    stuff = re.findall('^X-DSPAM-Confidence: ([0-9.]+)', line)
    if len(stuff) != 1 : continue
    num = float(stuff[0])
    numlist.append(num)
    print 'Maximum:', max(numlist)
python ds.py

X-DSPAM-Confidence: 0.8475


Escape Character


• If you want a special regular expression character to just behave normally (most of the time) you prefix it with '\'

>>> import re
>>> x = 'We just received $10.00 for cookies.'
>>> y = re.findall('\$[0-9.]+',x)
>>> print y
['$10.00']





Comments