Ensure CI matching operator receives an object

Ensure the evaluation of right-hand side expression always
results in the returning of an object or an empty String

changelogs/unreleased/64383-pattern-matching-with-variables-causes-gitlabs-ci-lint-to-throw-500.yml

+---
+title: Fix 500 errors caused by pattern matching with variables in CI Lint
+merge_request: 31719
+author:
+type: fixed

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

@@ -11,8 +11,9 @@ module Gitlab
             def evaluate(variables = {})
               text = @left.evaluate(variables)
               regexp = @right.evaluate(variables)
+              return false unless regexp
 
-              regexp.scan(text.to_s).any?
+              regexp.scan(text.to_s).present?
             end
 
             def self.build(_value, behind, ahead)

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

@@ -11,8 +11,9 @@ module Gitlab
             def evaluate(variables = {})
               text = @left.evaluate(variables)
               regexp = @right.evaluate(variables)
+              return true unless regexp
 
-              regexp.scan(text.to_s).none?
+              regexp.scan(text.to_s).empty?
             end
 
             def self.build(_value, behind, ahead)

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

@@ -69,6 +69,34 @@ describe Gitlab::Ci::Pipeline::Expression::Lexeme::Matches do
       it { is_expected.to eq(false) }
     end
 
+    context 'when right is nil' do
+      let(:left_value)  { 'my-awesome-string' }
+      let(:right_value) { nil }
+
+      it { is_expected.to eq(false) }
+    end
+
+    context 'when left and right are nil' do
+      let(:left_value)  { nil }
+      let(:right_value) { nil }
+
+      it { is_expected.to eq(false) }
+    end
+
+    context 'when left is an empty string' do
+      let(:left_value)  { '' }
+      let(:right_value) { Gitlab::UntrustedRegexp.new('pattern') }
+
+      it { is_expected.to eq(false) }
+    end
+
+    context 'when left and right are empty strings' do
+      let(:left_value)  { '' }
+      let(:right_value) { Gitlab::UntrustedRegexp.new('') }
+
+      it { is_expected.to eq(true) }
+    end
+
     context 'when left is a multiline string and matches right' do
       let(:left_value) do
         <<~TEXT

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

@@ -69,6 +69,34 @@ describe Gitlab::Ci::Pipeline::Expression::Lexeme::NotMatches do
       it { is_expected.to eq(true) }
     end
 
+    context 'when right is nil' do
+      let(:left_value)  { 'my-awesome-string' }
+      let(:right_value) { nil }
+
+      it { is_expected.to eq(true) }
+    end
+
+    context 'when left and right are nil' do
+      let(:left_value)  { nil }
+      let(:right_value) { nil }
+
+      it { is_expected.to eq(true) }
+    end
+
+    context 'when left is an empty string' do
+      let(:left_value)  { '' }
+      let(:right_value) { Gitlab::UntrustedRegexp.new('pattern') }
+
+      it { is_expected.to eq(true) }
+    end
+
+    context 'when left and right are empty strings' do
+      let(:left_value)  { '' }
+      let(:right_value) { Gitlab::UntrustedRegexp.new('') }
+
+      it { is_expected.to eq(false) }
+    end
+
     context 'when left is a multiline string and matches right' do
       let(:left_value) do
         <<~TEXT