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

• 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

• 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]

import re

hand = open('mbox-short.txt')

for line in hand:

    line = line.rstrip()

    if re.search('From:', line) :

        print line

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

• 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.*:

• 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

• 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

[]

• 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 :']

^F.+:

Last character in the match is a :

• 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.+?:

Last character in the match is a :

• 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']

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

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 '

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

• 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']