Regular Expressions 1#

To extract information from strings, we have been using string methods such as split() and find(), and lists and string slicing. However, the task of extracting data is so common that Python provides a library dedicated to handle these tasks in a more elegant manner. The library is called re (regular expressions) and it should be imported before you can start using it.

Regular expressions are like a little programming language created for searching and parsing strings.

There are general libraries, not Python specific, that offer this functionality as well (e.g. grep).

One of the simplest uses of the re library is the search() function, which verifies if a string is part of another string. Let us see an example.

file = open('datasets/mbox-short.txt')

for line in file:
    line: str = line.rstrip()
    print(line)

---------------------------------------------------------------------------
FileNotFoundError                         Traceback (most recent call last)
Cell In[1], line 1
----> 1 file = open('datasets/mbox-short.txt')
      3 for line in file:
      4     line: str = line.rstrip()

File /opt/hostedtoolcache/Python/3.8.15/x64/lib/python3.8/site-packages/IPython/core/interactiveshell.py:282, in _modified_open(file, *args, **kwargs)
    275 if file in {0, 1, 2}:
    276     raise ValueError(
    277         f"IPython won't let you open fd={file} by default "
    278         "as it is likely to crash IPython. If you know what you are doing, "
    279         "you can use builtins' open."
    280     )
--> 282 return io_open(file, *args, **kwargs)

FileNotFoundError: [Errno 2] No such file or directory: 'datasets/mbox-short.txt'

import re

file = open('datasets/mbox-short.txt')

for line in file:
    line: str = line.rstrip()
    if re.search('Author:', line):
        print(line)

Author: stephen.marquard@uct.ac.za
Author: louis@media.berkeley.edu
Author: zqian@umich.edu
Author: rjlowe@iupui.edu
Author: zqian@umich.edu
Author: rjlowe@iupui.edu
Author: cwen@iupui.edu
Author: cwen@iupui.edu
Author: gsilver@umich.edu
Author: gsilver@umich.edu
Author: zqian@umich.edu
Author: gsilver@umich.edu
Author: wagnermr@iupui.edu
Author: zqian@umich.edu
Author: antranig@caret.cam.ac.uk
Author: gopal.ramasammycook@gmail.com
Author: david.horwitz@uct.ac.za
Author: david.horwitz@uct.ac.za
Author: david.horwitz@uct.ac.za
Author: david.horwitz@uct.ac.za
Author: stephen.marquard@uct.ac.za
Author: louis@media.berkeley.edu
Author: louis@media.berkeley.edu
Author: ray@media.berkeley.edu
Author: cwen@iupui.edu
Author: cwen@iupui.edu
Author: cwen@iupui.edu

The previous program prints all the lines that have the word 'Author'.

As you might have noticed, we could implement this same program with the use of the find() method.

The power of the regular expressions comes when we add special characters to the search string, which allows us to more precisely control which lines match the string.

file = open('datasets/mbox-short.txt')

for line in file:
    line: str = line.rstrip()
    
    if re.search('^Author:', line):
        print(line)

We modified our previous program, so it only prints the lines that start with the 'Author' string. This is done thanks to the use of the caret character (^), which is used in regular expressions to match the beginning of a line.

Still, this example can also work with the use of the startswith() method, but along this chapter we will try to convince you that in some cases using regular expressions eases our lives.

Do It Yourself!

Add all lines that start by ‘Details:’ to a list. Use the search function.

# Remove this line and add your code here

Character Matching#

There are other characters that help us build powerful regular expressions. For instance, the period or full stop character matches any character within a string.

The regular expression s..r can match any 4-letter word that starts with s and ends with r, like scar, sear, slur, and soar.

Let us now print all lines that start by X- followed by four characters and ending with M.

file = open('datasets/mbox-short.txt')

for line in file:
    line: str = line.rstrip()
    
    if re.search('^X-....M', line):
        print(line)

This can be simplified by indicating that a character can be repeated multiple times. To do so, we use the * and the + wildcard characters.

The asterisk means that we need to match zero-or-more characters; while the plus sign states that we need to match one-or-more characters. The * or + applies to the single character placed immediately to their left.

The use of * and + results into a greedy match. A greedy match indicates that we match the longest possible string.

Let’s now print all lines that start with X-, that are followed by one-or-more characters and that then have the Result word.

file = open('datasets/mbox-short.txt')

for line in file:
    line: str = line.rstrip()
    
    if re.search('^X-.+Result', line):
        print(line)

Do It Yourself!

Print all lines that start by ‘Received:’, followed by zero-or-more charcaters, and that have the word ‘nakamura’.

# Remove this line and add your code here

Extracting Data#

We can also extract data from a string if we use the findall() method. This method will extract all substrings that match a given regular expression.

For instance, the findall() method is quite handy if we want to get all email addresses on the file, regardless of the format of the line where they are found. In this way, we do not need to write code for each type of line.

Let us see a small example before we create the whole program.

line: str = 'Einstein einstein@uni.edu sent an email to Ada ada@otheruni.edu about life @10'
emails: list = re.findall('\S+@\S+', line)
print(emails)

The findall() method returns a list of strings with all the words that match the regular expression \S+@\S+. \S matches a non-whitespace character.

Thus, the \S+@\S+ expression can be read as: one-or-more non-whitespace characters, followed by an @ character (at), followed by one-or-more non-whitespace characters.

Now, we are ready to create a program to gather all emails within the mbox-short.txt file.

file = open('datasets/mbox-short.txt')
emails: list = list()

for line in file:
    line: str = line.rstrip()
    line_emails = re.findall('\S+@\S+', line)
    emails.extend(line_emails)
    
emails

As you have noticed, some of the emails have additional characters that we would like to exclude (e.g. <, >, ;). We only want to extract the email.

To do so, we need to be more explicit when defining our regular expression. Thus, we will use square brackets to define the set of characters that we want to match at the beginning and end of the email.

The new regular expression looks like this: [a-zA-Z0-9]\S*@\S*[a-zA-Z]

  • The [a-zA-Z0-9] indicates that we are expecting one alphanumeric value (a letter between a-z or A-Z or a number between 0-9). We do not care if it is a capital or lower-case letter, that is why we add both a-z (for lower-case letters) and A-Z (for capital letters).

  • The [a-zA-Z] indicates that we will only match a lower-case or capital letter at the end of the string.

  • We switch to \S* because we already consider the need to match at least one character with the inclusion of both [a-zA-Z0-9] and [a-zA-Z].

file = open('datasets/mbox-short.txt')
emails: list = list()

for line in file:
    line: str = line.rstrip()
    line_emails: list = re.findall('[a-zA-Z0-9]\S*@[a-zA-Z.]+', line)
    emails.extend(line_emails)
    
emails

['stephen.marquard@uct.ac.za',
 'postmaster@collab.sakaiproject.org',
 '200801051412.m05ECIaH010327@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'stephen.marquard@uct.ac.za',
 'source@collab.sakaiproject.org',
 'stephen.marquard@uct.ac.za',
 'stephen.marquard@uct.ac.za',
 'louis@media.berkeley.edu',
 'postmaster@collab.sakaiproject.org',
 '200801042308.m04N8v6O008125@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'louis@media.berkeley.edu',
 'source@collab.sakaiproject.org',
 'louis@media.berkeley.edu',
 'louis@media.berkeley.edu',
 'zqian@umich.edu',
 'postmaster@collab.sakaiproject.org',
 '200801042109.m04L92hb007923@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'zqian@umich.edu',
 'source@collab.sakaiproject.org',
 'zqian@umich.edu',
 'zqian@umich.edu',
 'rjlowe@iupui.edu',
 'postmaster@collab.sakaiproject.org',
 '200801042044.m04Kiem3007881@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'rjlowe@iupui.edu',
 'source@collab.sakaiproject.org',
 'rjlowe@iupui.edu',
 'rjlowe@iupui.edu',
 'zqian@umich.edu',
 'postmaster@collab.sakaiproject.org',
 '200801042001.m04K1cO0007738@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'zqian@umich.edu',
 'source@collab.sakaiproject.org',
 'zqian@umich.edu',
 'zqian@umich.edu',
 'zqian@umich.edu',
 'rjlowe@iupui.edu',
 'postmaster@collab.sakaiproject.org',
 '200801041948.m04JmdwO007705@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'rjlowe@iupui.edu',
 'source@collab.sakaiproject.org',
 'rjlowe@iupui.edu',
 'rjlowe@iupui.edu',
 'cwen@iupui.edu',
 'postmaster@collab.sakaiproject.org',
 '200801041635.m04GZQGZ007313@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'cwen@iupui.edu',
 'source@collab.sakaiproject.org',
 'cwen@iupui.edu',
 'cwen@iupui.edu',
 'hu2@iupui.edu',
 'cwen@iupui.edu',
 'postmaster@collab.sakaiproject.org',
 '200801041633.m04GX6eG007292@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'cwen@iupui.edu',
 'source@collab.sakaiproject.org',
 'cwen@iupui.edu',
 'cwen@iupui.edu',
 'hu2@iupui.edu',
 'gsilver@umich.edu',
 'postmaster@collab.sakaiproject.org',
 '200801041611.m04GB1Lb007221@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'gsilver@umich.edu',
 'source@collab.sakaiproject.org',
 'gsilver@umich.edu',
 'gsilver@umich.edu',
 'gsilver@umich.edu',
 'postmaster@collab.sakaiproject.org',
 '200801041610.m04GA5KP007209@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'gsilver@umich.edu',
 'source@collab.sakaiproject.org',
 'gsilver@umich.edu',
 'gsilver@umich.edu',
 'zqian@umich.edu',
 'postmaster@collab.sakaiproject.org',
 '200801041609.m04G9EuX007197@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'zqian@umich.edu',
 'source@collab.sakaiproject.org',
 'zqian@umich.edu',
 'zqian@umich.edu',
 'gsilver@umich.edu',
 'postmaster@collab.sakaiproject.org',
 '200801041608.m04G8d7w007184@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'gsilver@umich.edu',
 'source@collab.sakaiproject.org',
 'gsilver@umich.edu',
 'gsilver@umich.edu',
 'wagnermr@iupui.edu',
 'postmaster@collab.sakaiproject.org',
 '200801041537.m04Fb6Ci007092@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'wagnermr@iupui.edu',
 'source@collab.sakaiproject.org',
 'wagnermr@iupui.edu',
 'wagnermr@iupui.edu',
 'zqian@umich.edu',
 'postmaster@collab.sakaiproject.org',
 '200801041515.m04FFv42007050@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'zqian@umich.edu',
 'source@collab.sakaiproject.org',
 'zqian@umich.edu',
 'zqian@umich.edu',
 'antranig@caret.cam.ac.uk',
 'postmaster@collab.sakaiproject.org',
 '200801041502.m04F21Jo007031@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'antranig@caret.cam.ac.uk',
 'source@collab.sakaiproject.org',
 'antranig@caret.cam.ac.uk',
 'antranig@caret.cam.ac.uk',
 'gopal.ramasammycook@gmail.com',
 'postmaster@collab.sakaiproject.org',
 '200801041403.m04E3psW006926@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'gopal.ramasammycook@gmail.com',
 'source@collab.sakaiproject.org',
 'gopal.ramasammycook@gmail.com',
 'gopal.ramasammycook@gmail.com',
 'david.horwitz@uct.ac.za',
 'postmaster@collab.sakaiproject.org',
 '200801041200.m04C0gfK006793@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'david.horwitz@uct.ac.za',
 'source@collab.sakaiproject.org',
 'david.horwitz@uct.ac.za',
 'david.horwitz@uct.ac.za',
 'david.horwitz@uct.ac.za',
 'dhorwitz@david',
 'dhorwitz@david',
 'david.horwitz@uct.ac.za',
 'postmaster@collab.sakaiproject.org',
 '200801041106.m04B6lK3006677@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'david.horwitz@uct.ac.za',
 'source@collab.sakaiproject.org',
 'david.horwitz@uct.ac.za',
 'david.horwitz@uct.ac.za',
 'david.horwitz@uct.ac.za',
 'postmaster@collab.sakaiproject.org',
 '200801040947.m049lUxo006517@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'david.horwitz@uct.ac.za',
 'source@collab.sakaiproject.org',
 'david.horwitz@uct.ac.za',
 'david.horwitz@uct.ac.za',
 'josrodri@iupui.edu',
 'dhorwitz@david',
 'david.horwitz@uct.ac.za',
 'postmaster@collab.sakaiproject.org',
 '200801040932.m049W2i5006493@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'david.horwitz@uct.ac.za',
 'source@collab.sakaiproject.org',
 'david.horwitz@uct.ac.za',
 'david.horwitz@uct.ac.za',
 'josrodri@iupui.edu',
 'dhorwitz@david',
 'stephen.marquard@uct.ac.za',
 'postmaster@collab.sakaiproject.org',
 '200801040905.m0495rWB006420@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'stephen.marquard@uct.ac.za',
 'source@collab.sakaiproject.org',
 'stephen.marquard@uct.ac.za',
 'stephen.marquard@uct.ac.za',
 'louis@media.berkeley.edu',
 'postmaster@collab.sakaiproject.org',
 '200801040023.m040NpCc005473@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'louis@media.berkeley.edu',
 'source@collab.sakaiproject.org',
 'louis@media.berkeley.edu',
 'louis@media.berkeley.edu',
 'louis@media.berkeley.edu',
 'postmaster@collab.sakaiproject.org',
 '200801032216.m03MGhDa005292@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'louis@media.berkeley.edu',
 'source@collab.sakaiproject.org',
 'louis@media.berkeley.edu',
 'louis@media.berkeley.edu',
 'ray@media.berkeley.edu',
 'postmaster@collab.sakaiproject.org',
 '200801032205.m03M5Ea7005273@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'ray@media.berkeley.edu',
 'source@collab.sakaiproject.org',
 'ray@media.berkeley.edu',
 'ray@media.berkeley.edu',
 'cwen@iupui.edu',
 'postmaster@collab.sakaiproject.org',
 '200801032133.m03LX3gG005191@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'cwen@iupui.edu',
 'source@collab.sakaiproject.org',
 'cwen@iupui.edu',
 'cwen@iupui.edu',
 'cwen@iupui.edu',
 'postmaster@collab.sakaiproject.org',
 '200801032127.m03LRUqH005177@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'cwen@iupui.edu',
 'source@collab.sakaiproject.org',
 'cwen@iupui.edu',
 'cwen@iupui.edu',
 'wagnermr@iupui.edu',
 'cwen@iupui.edu',
 'postmaster@collab.sakaiproject.org',
 '200801032122.m03LMFo4005148@nakamura.uits.iupui.edu',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'source@collab.sakaiproject.org',
 'apache@localhost',
 'source@collab.sakaiproject.org',
 'cwen@iupui.edu',
 'source@collab.sakaiproject.org',
 'cwen@iupui.edu',
 'cwen@iupui.edu',
 'wagnermr@iupui.edu']

Do It Yourself!

Use regular expressions to count all the times that the email ‘cwen@iupui.edu’ appears on the file regardles of the line where it is placed.

# Remove this line and add your code here

Combining Searching and Extracting#

Let us imagine that now we want to extract all the numbers that follow an X-Something: header, such as:

X-DSPAM-Confidence: 0.8475
X-DSPAM-Probability: 0.0000

To do so, we can define the following regular expression:

^X-.*: [0-9.]+

^X- indicates that the line should start with ‘X-’. Then, it must be followed by zero-or-more characters (.*), followed then by a colon and a space (: ). Finally, the string should be followed by a one-or-more numbers 0-9 that can have points . ([0-9.]+). Notice that when we use the . inside square brackets, it does not match any character but instead it matches literally the point character.

Before extracting the numbers, let us just print the lines that match our regular expression.

file = open('datasets/mbox-short.txt')

for line in file:
    line: str = line.rstrip()
    
    if re.search('^X-.*: [0-9.]+', line):
        print(line)
    
emails

We should use split() to actually get the decimal number out of our target lines. However, we will use regular expressions to achieve the same goal.

We introduce now the use of parentheses (). When you use parentheses in regular expressions, they are just completely ignored. You usually use them to group terms in your expression.

Nevertheless, if you use () in a regular expression inside the findall() method, they serve a different purpose: they indicate that you are interested in extracting only a portion of the string that matches the expression.

So, we modify our expression as follows:

^X-.*: ([0-9.]+)

file = open('datasets/mbox-short.txt')

for line in file:
    line: str = line.rstrip()
    match: list = re.findall('^X-.*: ([0-9.]+)', line)
    
    if len(match) > 0:
        print(match)

Let us explore another example. The mbox-short.txt file also has the following type of lines:

Details: http://source.sakaiproject.org/viewsvn/?view=rev&rev=39771

We would like to extract the revision number placed at the end of the string after rev=. In this example the revision number is 39771.

We will now extract these numbers all along the file.

file = open('datasets/mbox-short.txt')

for line in file:
    line: str = line.rstrip()
    revision: list = re.findall('^Details:.*rev=([0-9.]+)', line)
    
    if len(revision) > 0:
        print(revision)

We used the regular expression ^Details:.*rev=([0-9.]+).

^Details: indicates that the line should start by the word Details:; it will be then followed by zero-or-more characters (.*); and then it will find the rev= string. After the latter, we want to only extract the number that follows, which is composed by one-or-more numbers and points ([0-9.]+).

Remember that the [0-9]+ is greedy and it tries to make as large a string of digits as possible before extracting those digits. Because of this behaviour, we get all five digits for each number.

Do It Yourself!

Use regular expressions to extract the IP numbers of lines with the following format (the IP number is the number that you find between the square brackets at the end of the string, e.g. 141.211.14.46):

Received: from murder (mail.umich.edu [141.211.14.46])

# Remove this line and add your code here

Escape Character#

We need a way to use special characters within regular expressions in a normal or literal way. For instance, there might be cases where we want to match the caret ^ character, and not just use it to denote the start of a line. To do so, we use the escape character \ before we type the character we want to match (e.g. \^).

msg: str = 'This is how you write 2 to the power of 2 in Java: 2^2'
matches: list = re.findall('[0-9]+\^[0-9]+', msg)
matches

Remember that if you write special characters inside square brackets, characters are taken as literals. If we write [0-9.^] it literally means that we want to match digits (0-9) or the dot (.) or the caret character (^).

Do It Yourself!

Match all prices with the receipt string and return the sum of them all.

receipt: str = "Chocolate $2.0, Soap $10.0, Bread $1.4, Milk $2.4"

# Remove this line and add your code here

Regular Expressions Cheat Sheet#

In this section, we introduce some of the characters you can use when creating your regular expressions.

Special Characters#

Character

Description

^

Matches the start of a string

$

Matches the end of a string

.

Matches any character except new line characters (e.g. \n)

\

Escapes special characters

A|B

Matches expression A or B

+

Greedy match of one-or-more characters

*

Greedy match of zero-or-more characters

?

Greedy match of zero-or-one character. If added after a qualifier (i.e. +, *, ?) performs non-greedy (or lazy) matches

{n}

Matches an expression n times

{n, m}

Matches an expression from n to m times

Character Classes#

Character

Description

\w

Matches alphanumeric characters (i.e. a-z, A-Z, 0-9 and _)

\d

Matches digits (i.e. 0-9)

\D

Matches non-digits

\s

Matches whitespace characters (e.g. \t, \n, \r)

\S

Matches non-whitespace characters

\b

Matches the empty string, but only at the start or end of a word

\B

Matches the empty string, but not at the start or end of a word

Sets#

Character

Description

[ ]

Contains characters to match

[ab]

Matches character a or b, not ab

[a-z]

Matches a lower-case letter from a to z

[A-Z]

Matches a capital letter from A to Z

[0-9]

Matches a digit from 0 to 9

[a-zA-Z0-9]

Matches alphanumeric characters

[+*().]

Matches special characters as literals

[^ab]

Matches any character except a and b

Common re Methods#

Method

Description

re.findall(exp, string)

Matches substrings in string that match expression exp, and returns them on a list.

re.search(exp, string)

Matches the first substring in string that matches expression exp, and returns it as a match object.

re.split(exp, string)

Splits string into a list by means of using expression exp as delimiter.

re.sub(exp1, exp2, string)

Replaces all matches of expression exp1 with exp2 in the string.

Do It Yourself!

Write a program to look for lines of the form: ‘New Revision: 39772’. Then, extract all numers and add them up.

# Remove this line and add your code here
1

This Jupyter Notebook is based on Chapter 11 of [Sev16].