This chapter covers file I/O, string manipulation, and regular expressions. These are the workhorses of scripting — the tools you reach for when Ruby is solving a real problem rather than illustrating a concept.
Files
text = File.read("my_file.txt") # read entire file as string
File.open("my_file.txt") do |f| # block form — auto-closes
f.each_line { |line| puts line }
end
lines = File.readlines("my_file.txt") # array of lines
lines.each { |line| puts line.chomp }
File.write("output.txt", "hello world
") # write a file
String Methods (The Good Parts)
s = "Why the Lucky Stiff"
s.upcase # => "WHY THE LUCKY STIFF"
s.downcase # => "why the lucky stiff"
s.swapcase # => "wHY THE lUCKY sTIFF"
s.capitalize # => "Why the lucky stiff"
s.reverse # => "ffitS ykcuL ehT yhW"
s.length # => 19
s.chars.first(3) # => ["W", "h", "y"]
s.include?("Lucky") # => true
s.start_with?("Why") # => true
s.sub("Lucky", "Unlucky") # replaces first
s.gsub("l", "L") # replaces all
s.split(" ") # => ["Why", "the", "Lucky", "Stiff"]
s.strip # removes leading/trailing whitespace
s[4..6] # => "the" (substring by range)
Regular Expressions
text = "perplexity: 45.3, burstiness: 0.22" text.match(/perplexity: ([d.]+)/) # => MatchData — or nil if no match $1 # => "45.3" (first capture group from last match) text.scan(/[d.]+/) # => ["45.3", "0.22"] text.gsub(/d/, '*') # => "perplexity: **.*,..." text =~ /perplexity/ # => 0 (position) or nil text[/[d.]+/] # => "45.3" (first match)
Exception Handling
begin
result = 10 / 0
rescue ZeroDivisionError => e
puts "Caught: #{e.message}"
ensure
puts "This always runs"
end
# Custom exceptions
class DetectionError < StandardError; end
def detect_ai(text)
raise DetectionError, "text too short" if text.length < 50
# ... detection logic
end
begin
detect_ai("hi")
rescue DetectionError => e
puts "Detection failed: #{e.message}"
end
Iterators and Lazy Enumerators
require 'enumerator'
# lazy evaluation — only processes what you need
(1..Float::INFINITY).lazy
.select { |n| n % 3 == 0 }
.map { |n| n * n }
.first(5)
# => [9, 36, 81, 144, 225]