Gray Soft / Tags / ActiveRecord

11

NOV
2011

Doing it Wrong

Continuing with my Breaking All of the Rules series, I want to peek into several little areas where I've been caught doing the wrong thing. I'm a rule breaker and I'm determined to take someone down with me!

My Forbidden Parser

In one application, I work with an API that hands me very simple data like this:

<emails>
  <email>user1@example.com</email>
  <email>user2@example.com</email>
  <email>user3@example.com</email>
  …
</emails>

Now I need to make a dirty confession: I parsed this with a Regular Expression.

I know, I know. We should never parse HTML or XML with a Regular Expression. If you don't believe me, just take a moment to actually read that response. Yikes!

Oh and you shouldn't validate emails with a Regular Expression. Oops. We're talking about at least two violations here.

But it gets worse.

You may be think I rolled a little parser based on Regular Expressions. That might look like this:

#!/usr/bin/env ruby -w

require "strscan"

class EmailParser
  def initialize(data)
    @scanner = StringScanner.new(data)
  end

  def parse(&block)
    parse_emails(&block)
  end

  private

  def parse_emails(&block)
    @scanner.scan(%r{\s*<emails>\s*}) or fail "Failed to match list start"
    loop do
      parse_email(&block) or break
    end
    @scanner.scan(%r{\s*</emails>}) or fail "Failed to match list end"
  end

  def parse_email(&block)
    if @scanner.scan(%r{<email>\s*})
      if email = @scanner.scan_until(%r{</email>\s*})
        block[email.strip[0..-9].strip]
        return true
      else
        fail "Failed to match email end"
      end
    end
    false
  end
end

EmailParser.new(ARGF.read).parse do |email|
  puts email
end