ruby regex named and group

Question

I am trying to use a named group in a regex but it doesn't work:

module Parser
  def fill(line, pattern)
    if /\s#{pattern}\:\s*(\w+)\s*\;/ =~ line
      puts Regexp.last_match[1]
      #self.send("#{pattern}=", value)
    end
    if /\s#{pattern}\:\s*(?<value>\w+)\s*\;/ =~ line
      puts value
      #self.send("#{pattern}=", value)
    end
  end
end

As you can see I first test my regex then I try to use the same regex with a named group.

class Test
  attr_accessor :name, :type, :visible
  include Parser #add instance method (use extend if we need class method)
  def initialize(name)
    @name = name
    @type = "image"
    @visible = true
  end
end

t = Test.new("toto")
s='desciption{ name: "toto.test"; type: RECT; mouse_events: 0;'
puts t.type
t.fill(s, "type")
puts t.type

When I execute this, the first regex work but not the second with the named group. Here is the output:

./ruby_mixin_test.rb
image
RECT
./ruby_mixin_test.rb:11:in `fill': undefined local variable or method `value' for 
#<Test:0x00000001a572c8> (NameError)
from ./ruby_mixin_test.rb:34:in `<main>'

Arup Rakshit · Accepted Answer · 2014-02-21 11:52:06Z

7

If =~ is used with a regexp literal with named captures, captured strings (or nil) is assigned to local variables named by the capture names.

/(?<lhs>\w+)\s*=\s*(?<rhs>\w+)/ =~ "  x = y  "
p lhs    #=> "x"
p rhs    #=> "y"

But - A regexp interpolation, #{}, also disables the assignment.

rhs_pat = /(?<rhs>\w+)/
/(?<lhs>\w+)\s*=\s*#{rhs_pat}/ =~ "x = y"
lhs    # undefined local variable

In your case from the below code :

if /\s#{pattern}\:\s*(?<value>\w+)\s*\;/ =~ line
   puts value
   #self.send("#{pattern}=", value)
end

Look at the line below, you use interpolation

/\s#{pattern}\:\s*(?<value>\w+)\s*\;/ =~ line
 ~~^

Thus local variable assignment didn't happen and you got the error as you reported undefined local variable or method 'value'.

edited Feb 21, 2014 at 11:52

answered Feb 21, 2014 at 11:45

Arup Rakshit

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2 Comments

cedlemo Over a year ago

your answer is very ineresting. Have you some sources please?

Dan Over a year ago

If you are looking for a solution, then it seems from tinkering that although lhs is not set, $~[:lhs] is, so perhaps that is the best workaround?

Santhosh · Accepted Answer · 2014-02-21 11:32:19Z

1

You have not defined value in the module

if /\s#{pattern}\:\s*(?<value>\w+)\s*\;/ =~ line
  puts value  # This is not defined anywhere
  [..]

answered Feb 21, 2014 at 11:32

Santhosh

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