teaching machines

CS 330 Lecture 4 – Regular Expressions

January 29, 2014 by Chris Johnson. Filed under cs330, lectures, spring 2014.

Agenda

• what ?s
• the gauntlet
• state machines/DFAs
• DFAs are regular expressions
• bingo

TODO

• Walk through http://regexone.com.
• Write expressions to match:
  • Lines ending with a hyphenated word.
  • Words with an internal uppercase letter.
  • Lines lacking a semi-colon at their close. Don’t match lines that have a semi-colon followed by whitespace.
  • Instances of the identifier i. Don’t match other occurrences of i.
• 1/4 sheet.

While You’re Waiting

• We’re going to play some regex bingo later on. You and a partner make a 3×3 grid of randomly generated strings. Include upper- and lowercase letters, numbers, whitespace, and punctuation. Keep the strings short. There’s no free space.

State Machines

Code

show_matches.rb

#!/usr/bin/env ruby

txt = IO.read('eg.txt')
puts txt.scan(/[0-9]/)
puts '--------------'
puts txt.scan(/\S+@\S+/)
puts '--------------'
puts txt.scan(/\.\s+[a-z]/)
puts '--------------'
puts txt.scan(/<b>.*<\/b>/)
puts '--------------'
puts txt.scan(/<.*?>/)

eg.txt

asdf asdf 1 234 456
asdfkl sdf kdfklads f <b>56</b>
johnch@uwec.edu
asdfk foo@foo.com
The dog is <b>luscious</b>. The cat is blah. those who walk on rainbows peak at middle age.

Haiku