Discussion 11

Regular Expressions 🥳

anto [at] berkeley [dot] edu

All slides can be found on

Slides by Antonio Kam (anto@)


  • Scheme Project Phase 1 due today
  • Phases 2 and 3 due Friday
  • Final project due on Tuesday
  • Would recommend getting the whole project done by the weekend
  • I'm ending today's discussion slightly earlier so I can go to Dwinelle!
    • come with me pls thx
Slides by Antonio Kam (anto@)

Notes from last section

  • Avid 3b1b watcher?
    • yep
    • got me through linalg/calc
  • your explanation of tail calls/recursion was so good!! made me understand the lecture so much better
    • honestly a shocker because i thought i crapped the bed
  • Python >> Scheme
    • i strongly agree
Slides by Antonio Kam (anto@)

Notes from last section

  • What notes app do you use on your ipad?
    • Goodnotes
    • Used to use OneNote/Microsoft Journal back when I used a surface/windows though
  • In the game Pokemon, there is a tactic known as Fear. this is an acronym, standing for Focus sash, Endeavor, (quick) Attack, and Rattata. [...]
    • I personally liked the shell bell/aron/sandstorm thing a lot more just cause I thought it was funnier and could potentially last longer
    • There was also a FEAR reference in the Fall 2022 Final
    • fun question
Slides by Antonio Kam (anto@)

Temperature Check 🌡️

  • RegEx
Slides by Antonio Kam (anto@)

RegEx 👀

Poll: How do we all pronounce it?

Slides by Antonio Kam (anto@)


  • Regular Expressions are used to describe sets of strings that meet certain criteria
  • Very useful for pattern matching (will see examples later)
    • Used in real life all the time!
    • Can be used for super simple patterns (egg in text)
    • Can also be very complicated (validating passwords)
  • regex101.com
    • Super useful for learning/debugging your regular expressions, but don't rely on this!
    • (You won't have this resource for the exam! Don't use it as a GPS)
Slides by Antonio Kam (anto@)

Character Classes

  • If you want to search for a certain character in a set of characters, use character classes
  • Can use pre-defined classes, or specify your own


Pattern Description
[abc] Matches either a, b, or c
[a-z] Matches anything from lowercase a to lowercase z
[^a-z] Matches anything that isn't from lowercase a to lowercase z
[0-9] Matches any (single) digit
[a-zA-Z123] Matches any lowercase or uppercase character, or the digits 1, 2, or 3
Slides by Antonio Kam (anto@)

Character Classes

Predefined character classes

Pattern Equivalent
\w [A-Za-z0-9_]
\d [0-9]
\s (Matches any 'whitespace' character)
. Matches everything except for newlines.
Slides by Antonio Kam (anto@)

Combining Multiple Patterns

  • If you put a pattern after another pattern, you can have a regular expression that matches both of those characters in succession
    • AB - Matches A, then B (both are required)
    • A[xt]e Matches A, then either x or t, then E
      • Axe or Ate are the things that get matched
  • | is the separator used to match 'or'
    • A|B - Matches either A or B
Slides by Antonio Kam (anto@)


Quantifier Description
? 0 or 1 (kind of like a question - is it there or not?)
* 0 or more
+ 1 or more
{a} Match exactly a times (number)
{a,} Match from a to infinity number of times
{0,b} Match from 0 to b number of times
{a,b} Match between a and b number of times
Slides by Antonio Kam (anto@)


  • Parentheses are used to group certain patterns together
    • Useful with quantifiers
    • (Ha)+ and Ha+ do different things!
Slides by Antonio Kam (anto@)


Anchor Description
^ Matches the beginning of the string
$ Matches the end of a string
\b Matches a word boundary
Slides by Antonio Kam (anto@)

Special Characters

Some characters cannot be written directly because they have special roles in RegEx

  • \[](){}+*?|$^.
  • Inside a character class, you need to escape -
  • To escape these characters, you need to put a backslash before them
    • The quick brown fox jumped over the lazy dog\.
Slides by Antonio Kam (anto@)

RegEx in Python

  • For Python, we use r-strings to write regex patterns
  • With r-strings in Python, backslashes are treated specially in some cases (for example with \n, or \t). What r-strings do in this case is that they automatically escape the backslashes for these special characters, so they look like real backslashes rather than what Python designates for those characters.
  • r"<insert string here>" - the r at the front makes a string an r-string
    • Python also has f-strings which are pretty useful/interesting
Slides by Antonio Kam (anto@)


Slides by Antonio Kam (anto@)

Results from last section (links.rouxl.es/disc)

Slides by Antonio Kam (anto@)

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Slides by Antonio Kam (anto@)

Anonymous Feedback Form

Thanks for coming! 🥳

Please give me feedback on what to improve!

Slides by Antonio Kam (anto@)