Regular Expressions


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Learning Group Activity

You should have watched either OOP is Bad or Stop Writing Classes before class. Discuss with your group:

  1. Summarize each video
  2. In your own opinion, what is OOP useful for? State some example use cases.
  3. In your own opinion, when might OOP convolute a code base (if ever)? State some examples.
  4. In your own opinion, when should we write classes? When shouldn’t we?
  5. In your own opinion, how should we design programming languages, taking into account potential issues of OOP, imperative programming, or functional programming.


I really do mean in your own opinion. The videos you watch were particularly opinionated, and I hope you took the opinion with a grain of salt.

Regular Expressions

Regular expression languages describe a search pattern on a string.

  • They are called regular, since they implement a regular language: a language which can be described using a finite state machine.
  • Typically used for determining if a string matches a pattern, replacing a pattern in a string, or extracting information from a string.
  • Regular expression languages are a family of languages, rather than just a single language. Many modern regular expression languages were inspired by Perl’s regular expression syntax.

Python’s Regular Expressions

Python’s regular expression language can be accessed using the re module:

>>> import re

Regular expressions can be compiled using re.compile. This returns a regular expression object:

>>> p = re.compile(r'ab[cd]')

There’s a number of things we might want to do with p here:

  • p.match: Match the beginning of a string
  • p.fullmatch: Match the whole string, without allowing characters at the end
  • Match anywhere in the string
  • p.finditer: Iterate over all of the matches in the string

Character Sets

  • [abcd] is a character set. It matches a single a, b, c, or d, only once.
  • Character sets also support a shorthand for ranges of characters, for example:
    • [0-9] matches a single digit
    • [a-z] matches a lowercase letter
    • [A-Z] matches an uppercase letter
  • These can even be combined:
    • [a-zA-Z2] will match a single lowercase letter, uppercase letter, or the digit 2.
  • A ^ (caret) at the beginning of a character set negates the set:
    • [^0-9] will match a single character that is not a digit.

Special Character Sets

As a convenience, Python gives us access to a few nice character sets:

  • \s matches any whitespace character
  • \S matches any non-whitespace character
  • \d matches any digit
  • \D matches any non-digit
  • \w matches any “word” character (capital letters, lowercase letters, digits, and underscores)
  • \W matches any non-word character

Any character

The . matches any character, exactly once.

  • will match tick, tock, and tuck, but not truck.

To match a literal period, write “\.”.

Match Objects

When we call match, fullmatch, or search, we get back a match object, or None if it did not match. When we iterate over finditer, we iterate on all of the match objects found.

>>> p = re.compile(r'[cd][ao][tg]')
>>> for word in 'cat', 'dog', 'cog', 'dat', 'datt':
...     print(bool(p.match(word)))
>>> for word in 'orange', 'apple', 'datum':
...     print(bool(p.match(word)))

How Many?

Often times, we want to match the previous group a certain number of times:

  • ? will match 0 or 1 times
  • + will match 1 or more times
  • * will match 0 or more times
  • {n} will match n times, exactly
  • {m,n} will match between m and n times

For example:

  • a?b matches ab as well as b
  • [A-Z]* matches any amount of capital letters, including none at all
  • [0-9]+ matches one or more digits
  • .* matches any character, zero or more times


Grouping allows us to:

  • Specify groups of characters to repeat
  • Alternate on different sets of characters
  • Capture the matched group and retrieve it in our match object

Groups are written in parentheses, and alternation is specified using a vertical bar (|):

  • Thanks?( you)? matches:
    • Thanks
    • Thank
    • Thank you
    • Thanks you
  • Thank(s| you) matches:
    • Thanks
    • Thank you

Grouping: Using Captures

On our match objects, we can obtain the result of a capture by calling .group:

>>> p = re.compile(r'My name is (\w+) and I like (\w+)')
>>> m = p.match('My name is Jack and I like computers')
>>>      # the whole match
'My name is Jack and I like computers'

Non-capturing Groups

Groups which begin with ?: are non-capturing groups. This means that they will not provide any visible group in the match object:

>>> p = re.compile(r'My name is (\w+)(?:, | and) I like (\w+)')
>>> m = p.match('My name is Jack and I like computers')
>>> m = p.match('My name is Jack, I like computers')


+, *, and ? are called greedy operators since they will try and match as many characters as possible, this may lead to undesired results:

>>> p = re.compile(r'#(.*)#')
>>> for m in p.finditer('#hello# a b c #world#'):
...     print(
hello# a b c #world

If we wanted to match as little as possible, we can use the non-greedy version of the operator, which would be +?, *?, or ??.

>>> p = re.compile(r'#(.*?)#')
>>> for m in p.finditer('#hello# a b c #world#'):
...     print(


Anchors match a certain kind of occurrence in a string, but not necessarily any characters.

  • ^ anchors to the beginning of a string, or to the beginning of a line when re.MULTILINE is passed to re.compile
  • $ anchors to the end of a string, or to the end of a line when re.MULTILINE is passed to re.compile
  • \b anchors to the boundary of a word: the transition from a \w to a \W, or visa versa. Also anchors to the beginning or end of a string.


  • foo\b.* matches foo and foo-dle, but not foodle
  • ^$ matches the empty string
  • //.*(\n$|$) matches // hello and // hello\n, but not // hello\n\n

Tip: Making Long REs Readable

Sometimes, when regular expressions get long, you need a way to comment them and break up sections to let other programmers (or yourself) know what’s going on.

When you pass re.VERBOSE to re.compile, whitespaces are ignored, and # starts a comment until the end of line:

p = re.compile(r'''
    (\w+)                          # first name
    (\w+)                          # last name
    ([2-9]\d{2}-[2-9]\d{2}-\d{4})  # phone number
''', re.VERBOSE)

RE Examples, and any Questions?

  • Matching a decimal number:

  • Matching a C/C++ identifier:

  • Matching a Mines Email address:


Finite State Machines

A finite state machine is any machine which has a finite number of states, and can only be in one state at a time. The machine has transitions that move it from one state to another.


A state diagram for your home phone

Regular Expressions as Finite State Machines

Regular expressions can be represented as finite state machines as well. Consider the following regular expression:


This matches both free and fee, we can write this in a state diagram like this:


Required Formalisms

  • Any state which could be a terminating state should be placed in double circles.
  • The transitions have the letters on them. The states do not.

Another Example: C/C++ identifiers

Recall the regular expression for C and C++ identifiers:


Translating REs to State Diagrams

With your learning group, translate each of these REs to a state diagram:

  • [A-Z]+
  • [A-Z]?x (hint: use \(\epsilon\) for the “no character” transition)
  • ([A-Z][1-5])+ (hint: draw a transition going backwards)

Write your names on your paper and turn in for bonus learning group participation points.