Regular Expressions


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Learning Group Activity

You should have researched one of these topics on the LGA:

  • Reference Couting
  • Smart Pointers
  • Valgrind

Explain to your group!

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.
  • Transitions correspond to only a single character, so repetition and groups must be encoded using the FSA.

Another Example: C/C++ identifiers

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



This is an open source tool developed by Sam Sartor (took CSCI-400 last semester) to help you visualize regular expressions using finite state graphs:

Translating REs to State Diagrams

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

  • [A-Z]+
  • [A-Z]?x (try using \(\epsilon\) 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.