Make variables expression pattern case-sensitivity explicit

doc/ci/variables/README.md

@@ -537,6 +537,12 @@ Below you can find supported syntax reference:
     It is possible perform pattern matching against a variable and regular
     expression. Expression like this evaluates to truth if matches are found.
 
+    Pattern matching is case-sensitive by default. Prepend `(?i)` to a
+    match-group to make a pattern case-insensitive.
+
+    Under the hood we are using [RE2 library][re2-library], see
+    [syntax documentation][re2-syntax] for reference.
+
 ### Unsupported predefined variables
 
 Because GitLab evaluates variables before creating jobs, we do not support a
@@ -577,3 +583,5 @@ These variables are also not supported in a context of a
 [builds-policies]: ../yaml/README.md#only-and-except-complex
 [dynamic-environments]: ../environments.md#dynamic-environments
 [gitlab-deploy-token]: ../../user/project/deploy_tokens/index.md#gitlab-deploy-token
+[re2-library]: https://github.com/google/re2
+[re2-syntax]: https://github.com/google/re2/wiki/Syntax

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

@@ -63,5 +63,18 @@ describe Gitlab::Ci::Pipeline::Expression::Lexeme::Matches do
 
       expect(operator.evaluate).to eq true
     end
+
+    it 'supports regexp flags' do
+      allow(left).to receive(:evaluate).and_return <<~TEXT
+        My AWESOME content
+      TEXT
+
+      allow(right).to receive(:evaluate)
+        .and_return(Gitlab::UntrustedRegexp.new('(?i)awesome'))
+
+      operator = described_class.new(left, right)
+
+      expect(operator.evaluate).to eq true
+    end
   end
 end

spec/lib/gitlab/ci/pipeline/expression/statement_spec.rb

@@ -100,6 +100,7 @@ describe Gitlab::Ci::Pipeline::Expression::Statement do
       "$PRESENT_VARIABLE =~ /^var.*/"      | false
       "$EMPTY_VARIABLE =~ /var.*/"         | false
       "$UNDEFINED_VARIABLE =~ /var.*/"     | false
+      "$PRESENT_VARIABLE =~ /(?i)VAR.*/"   | true
     end
 
     with_them do