Implement variables expression untrusted pattern lexeme

lib/gitlab/ci/pipeline/expression/lexeme/pattern.rb

+module Gitlab
+  module Ci
+    module Pipeline
+      module Expression
+        module Lexeme
+          class Pattern < Lexeme::Value
+            PATTERN = %r{/(?<regexp>.+)/}.freeze
+
+            def initialize(regexp)
+              @value = regexp
+            end
+
+            def evaluate(variables = {})
+              Gitlab::UntrustedRegexp.new(@value.to_s)
+              # TODO raise LexerError / ParserError in case of RegexpError
+            end
+
+            def self.build(string)
+              new(string.match(PATTERN)[:regexp])
+            end
+          end
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/ci/pipeline/expression/lexeme/pattern_spec.rb

+require 'fast_spec_helper'
+require_dependency 're2'
+
+describe Gitlab::Ci::Pipeline::Expression::Lexeme::Pattern do
+  describe '.build' do
+    it 'creates a new instance of the token' do
+      expect(described_class.build('/.*/'))
+        .to be_a(described_class)
+    end
+  end
+
+  describe '.type' do
+    it 'is a value lexeme' do
+      expect(described_class.type).to eq :value
+    end
+  end
+
+  describe '.scan' do
+    it 'correctly identifies a pattern token' do
+      scanner = StringScanner.new('/pattern/')
+
+      token = described_class.scan(scanner)
+
+      expect(token).not_to be_nil
+      expect(token.build.evaluate)
+        .to eq Gitlab::UntrustedRegexp.new('pattern')
+    end
+
+    it 'is a greedy scanner for regexp boundaries' do
+      scanner = StringScanner.new('/some .* / pattern/')
+
+      token = described_class.scan(scanner)
+
+      expect(token).not_to be_nil
+      expect(token.build.evaluate)
+        .to eq Gitlab::UntrustedRegexp.new('some .* / pattern')
+    end
+
+    it 'does not allow to use an empty pattern' do
+      scanner = StringScanner.new(%(//))
+
+      token = described_class.scan(scanner)
+
+      expect(token).to be_nil
+    end
+  end
+
+  describe '#evaluate' do
+    it 'returns a regular expression' do
+      string = described_class.new('abc')
+
+      expect(string.evaluate).to eq Gitlab::UntrustedRegexp.new('abc')
+    end
+  end
+end