diff --git a/CHANGELOG.md b/CHANGELOG.md
index bb84ac8..1e169c7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Fixed
+- Use RegExp.source to represent regular expression in messages ([#12](https://github.com/cucumber/javascript-core/pull/12))
 
 ## [0.4.0] - 2025-08-26
 ### Added
diff --git a/src/SupportCodeBuilderImpl.ts b/src/SupportCodeBuilderImpl.ts
index 348a2ef..22d06db 100644
--- a/src/SupportCodeBuilderImpl.ts
+++ b/src/SupportCodeBuilderImpl.ts
@@ -115,6 +115,7 @@ export class SupportCodeBuilderImpl implements SupportCodeBuilder {
         if (!compiled) {
           return undefined
         }
+        const source = this.extractPatternSource(pattern)
         return {
           id,
           expression: {
@@ -131,7 +132,7 @@ export class SupportCodeBuilderImpl implements SupportCodeBuilder {
                   this.expression.compiled instanceof CucumberExpression
                     ? StepDefinitionPatternType.CUCUMBER_EXPRESSION
                     : StepDefinitionPatternType.REGULAR_EXPRESSION,
-                source: pattern.toString(),
+                source,
               },
               sourceReference,
             }
@@ -141,6 +142,13 @@ export class SupportCodeBuilderImpl implements SupportCodeBuilder {
       .filter((step) => !!step)
   }
 
+  private extractPatternSource(pattern: string | RegExp) {
+    if (pattern instanceof RegExp) {
+      return pattern.flags ? pattern.toString() : pattern.source
+    }
+    return pattern
+  }
+
   private compileExpression(
     text: string | RegExp
   ): CucumberExpression | RegularExpression | undefined {
diff --git a/src/buildSupportCode.spec.ts b/src/buildSupportCode.spec.ts
index be03ab7..a1fbf79 100644
--- a/src/buildSupportCode.spec.ts
+++ b/src/buildSupportCode.spec.ts
@@ -226,7 +226,41 @@ describe('buildSupportCode', () => {
           stepDefinition: {
             id: '0',
             pattern: {
-              source: '/there are (\\d+) widgets/',
+              source: 'there are (\\d+) widgets',
+              type: StepDefinitionPatternType.REGULAR_EXPRESSION,
+            },
+            sourceReference: {
+              location: {
+                column: 1,
+                line: 1,
+              },
+              uri: 'steps.js',
+            },
+          },
+        },
+      ])
+    })
+
+    it('handles regular expressions with flags', () => {
+      const library = buildSupportCode({ newId })
+        .step({
+          pattern: /there are (\d+) widgets/i,
+          fn: sinon.stub(),
+          sourceReference: { uri: 'steps.js', location: { line: 1, column: 1 } },
+        })
+        .build()
+      const matchedSteps = library.findAllStepsBy('there ARE 17 widgets')
+
+      expect(matchedSteps.length).to.eq(1)
+      expect(matchedSteps[0].def.expression.compiled).to.be.instanceof(RegularExpression)
+      expect(matchedSteps[0].args.length).to.eq(1)
+      expect(matchedSteps[0].args[0].getValue(undefined)).to.eq(17)
+      expect(library.toEnvelopes()).to.deep.eq([
+        {
+          stepDefinition: {
+            id: '0',
+            pattern: {
+              source: '/there are (\\d+) widgets/i',
               type: StepDefinitionPatternType.REGULAR_EXPRESSION,
             },
             sourceReference: {