» CS 330 Lecture 5

CS 330 Lecture 5 – Regex

February 3, 2016 by Chris Johnson. Filed under cs330, lectures, spring 2016.

Agenda

what ?s
regex anatomy
regex operations
- assert match (=~)
- find match (capturing groups)
- find all matches (scan, grep)
- replace matches (sub, gsub)

TODO

Walk through RegexOne.
On a 1/4 sheet, draft some regular expressions (and just the expressions) that match the following:
- 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.
Start the regexercise homework. Pull from both repos! Due before February 18.

Note

With a little Ruby under our belt, we can now dive into talking about regular expressions or regex. What is regex? A language for describing patterns in strings. A simple find/replace of literal text is not sufficient for problems where we don’t know exactly what we’re looking for. For instance, on my phone, I am always hitting the period key instead of the space key, yielding text like “The.quick.brown.fox.” It’d be nice to be able to locate these faux periods and replace them with a space. If all I can do is search for literal text, I have a lot of work cut out for me.

Why study regex in a Programming Languages class? I can think of a few reasons:

Regexes are frequently used by tools like compilers and syntax highlighters to break up source code into its parts and pieces.
The general goal of most programs is to set up a software machine that translates input into some sort of output. The information age that we live in means that a lot of this input is big and clunky, and regex is a tool for parsing that input into workable units.
Personally, I use regex quite a bit in editing my own source code.

Regexes can be broken down according to the following anatomy:

what to match (literals, character classes, wildcards)
how many to match (quantifiers)
where to match (anchors)

We’ll solve a few problems that benefit from regex! Today we’ll stick with just asserting that text matches a pattern and optionally capturing portions of the matching text.

Code

isidentifier

/usr/lib/ruby/2.7.0/rubygems/dependency.rb:311:in `to_specs': Could not find 'coderay' (>= 0) among 56 total gem(s) (Gem::MissingSpecError)
Checked in 'GEM_PATH=/.gem/ruby/2.7.0:/var/lib/gems/2.7.0:/usr/lib/ruby/gems/2.7.0:/usr/share/rubygems-integration/2.7.0:/usr/share/rubygems-integration/all:/usr/lib/x86_64-linux-gnu/rubygems-integration/2.7.0:/home/johnch/.gems', execute `gem env` for more information
	from /usr/lib/ruby/2.7.0/rubygems/dependency.rb:323:in `to_spec'
	from /usr/lib/ruby/2.7.0/rubygems/core_ext/kernel_gem.rb:62:in `gem'
	from ./coderay:24:in `'

govts

/usr/lib/ruby/2.7.0/rubygems/dependency.rb:311:in `to_specs': Could not find 'coderay' (>= 0) among 56 total gem(s) (Gem::MissingSpecError)
Checked in 'GEM_PATH=/.gem/ruby/2.7.0:/var/lib/gems/2.7.0:/usr/lib/ruby/gems/2.7.0:/usr/share/rubygems-integration/2.7.0:/usr/share/rubygems-integration/all:/usr/lib/x86_64-linux-gnu/rubygems-integration/2.7.0:/home/johnch/.gems', execute `gem env` for more information
	from /usr/lib/ruby/2.7.0/rubygems/dependency.rb:323:in `to_spec'
	from /usr/lib/ruby/2.7.0/rubygems/core_ext/kernel_gem.rb:62:in `gem'
	from ./coderay:24:in `'

emails

/usr/lib/ruby/2.7.0/rubygems/dependency.rb:311:in `to_specs': Could not find 'coderay' (>= 0) among 56 total gem(s) (Gem::MissingSpecError)
Checked in 'GEM_PATH=/.gem/ruby/2.7.0:/var/lib/gems/2.7.0:/usr/lib/ruby/gems/2.7.0:/usr/share/rubygems-integration/2.7.0:/usr/share/rubygems-integration/all:/usr/lib/x86_64-linux-gnu/rubygems-integration/2.7.0:/home/johnch/.gems', execute `gem env` for more information
	from /usr/lib/ruby/2.7.0/rubygems/dependency.rb:323:in `to_spec'
	from /usr/lib/ruby/2.7.0/rubygems/core_ext/kernel_gem.rb:62:in `gem'
	from ./coderay:24:in `'