• Regular Expression Functions

    PHP implements both POSIX and Perl-compatible regular expressions. The Perl-compatible regexp functions (which includes all the preg_* functions) is the preferred library for most developers since it has many features not available in POSIX, and is binary safe.

    Ruby uses Perl-compatible regular expressions, so if you’re familiar with the preg_* functions in PHP, you’re already well on your way to learning regular expressions in Ruby. Regular expressions are a complex topic, so we won’t be covering regular expression basics, but will instead focus on translating existing knowledge of Perl-compatible PHP functions to Ruby.

    Regular Expressions in Ruby

    We use regular expression patterns in PHP by passing a string argument to various functions. Ruby treats regular expressions differently. Instead of specifying the pattern within a string, they are objects just like everything else in Ruby.

    PHP

    $myRegexp = '/[a-z0-9]+\s/mi';
print gettype($myRegexp); 
// => string

    Ruby

    my_regexp = /[a-z0-9]+\s/mi
p my_regexp.class
# => Regexp

    We can create regular expressions in Ruby using two different literal syntaxes.
    The most common is by enclosing the pattern in forward-slashes, but we can also use an alternate %r{} syntax. We usually use %r{} when the pattern contains a lot of forward-slashes (such as a filepath). Regular expressions can also be explicitly instantiated using the Regexp class.

    Ruby

    /[a-z0-9]+\s/mi
%r{/path/to/gif\.gif}mi
Regexp.new("[a-z0-9]+\s", Regexp::IGNORECASE | Regexp::MULTILINE)

    Regular Expressions in Rails

    Rails uses regular expressions in various places to specify patterns. When we are matching a route in Rails, we can use them to assign a requirement that a route component must match:

    Rails

    ActionController::Routing::Routes.draw do |map|
  map.connect 'teams/:team_id/players/:action/:id, :team_id => /\d+/
end

    We can also use regular expressions in our models when we validate the format of data. We pass a regexp to the :with option of validates_format_of:

    Rails

    class Image < ActiveRecord::Base
  validates_format_of :url, :with => /\.(gif|jpg)/i, 
                      :message => "must be a GIF or JPG" 
end

    When we are testing controller code, the assert_select method will accept a regular expression to match response data according to the given pattern.

    Rails

    class HomepageControllerTest < ActionController::TestCase
  def test_greeting
    get :index
    assert_select 'div.greeting', /Welcome [a-z0-9-_]+/
  end
end

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