diff --git a/packages/instrumenter/package.json b/packages/instrumenter/package.json
index 00a9d7e059..6f6354af33 100644
--- a/packages/instrumenter/package.json
+++ b/packages/instrumenter/package.json
@@ -40,7 +40,7 @@
     "@stryker-mutator/api": "4.5.0",
     "@stryker-mutator/util": "4.5.0",
     "angular-html-parser": "~1.7.0",
-    "weapon-regex": "~0.3.0"
+    "weapon-regex": "~0.4.1"
   },
   "devDependencies": {
     "@babel/preset-react": "~7.12.1",
diff --git a/packages/instrumenter/src/mutators/regex-mutator.ts b/packages/instrumenter/src/mutators/regex-mutator.ts
index 00f1fa5fae..3693568a9c 100644
--- a/packages/instrumenter/src/mutators/regex-mutator.ts
+++ b/packages/instrumenter/src/mutators/regex-mutator.ts
@@ -22,25 +22,14 @@ function isObviousRegexString(path: NodePath<types.StringLiteral>) {
 }
 const weaponRegexOptions: weaponRegex.Options = { mutationLevels: [1] };
 
-function mutatePattern(pattern: string): string[] {
-  if (pattern.length) {
-    try {
-      return weaponRegex.mutate(pattern, weaponRegexOptions).map((mutant) => mutant.pattern);
-    } catch (err) {
-      console.error(
-        `[RegexMutator]: The Regex parser of weapon-regex couldn't parse this regex pattern: "${pattern}". Please report this issue at https://github.com/stryker-mutator/weapon-regex/issues. Inner error: ${err.message}`
-      );
-    }
-  }
-  return [];
-}
-
 export class RegexMutator implements NodeMutator {
   public name = 'Regex';
 
+  constructor(private readonly weaponRegexMutateImpl = weaponRegex.mutate) {}
+
   public mutate(path: NodePath): NodeMutation[] {
     if (path.isRegExpLiteral()) {
-      return mutatePattern(path.node.pattern).map((replacementPattern) => {
+      return this.mutatePattern(path.node.pattern).map((replacementPattern) => {
         const replacement = types.cloneNode(path.node, false);
         replacement.pattern = replacementPattern;
         return {
@@ -49,7 +38,7 @@ export class RegexMutator implements NodeMutator {
         };
       });
     } else if (path.isStringLiteral() && isObviousRegexString(path)) {
-      return mutatePattern(path.node.value).map((replacementPattern) => {
+      return this.mutatePattern(path.node.value).map((replacementPattern) => {
         const replacement = types.cloneNode(path.node, false);
         replacement.value = replacementPattern;
         return {
@@ -60,4 +49,17 @@ export class RegexMutator implements NodeMutator {
     }
     return [];
   }
+
+  private mutatePattern(pattern: string): string[] {
+    if (pattern.length) {
+      try {
+        return this.weaponRegexMutateImpl(pattern, weaponRegexOptions).map((mutant) => mutant.pattern);
+      } catch (err) {
+        console.error(
+          `[RegexMutator]: The Regex parser of weapon-regex couldn't parse this regex pattern: "${pattern}". Please report this issue at https://github.com/stryker-mutator/weapon-regex/issues. Inner error: ${err.message}`
+        );
+      }
+    }
+    return [];
+  }
 }
diff --git a/packages/instrumenter/test/unit/mutators/regex-mutator.spec.ts b/packages/instrumenter/test/unit/mutators/regex-mutator.spec.ts
index 231537ebad..2b97e1876a 100644
--- a/packages/instrumenter/test/unit/mutators/regex-mutator.spec.ts
+++ b/packages/instrumenter/test/unit/mutators/regex-mutator.spec.ts
@@ -5,36 +5,44 @@ import { RegexMutator } from '../../../src/mutators/regex-mutator';
 import { expectJSMutation } from '../../helpers/expect-mutation';
 
 describe(RegexMutator.name, () => {
-  let sut: RegexMutator;
-  beforeEach(() => {
-    sut = new RegexMutator();
-  });
-
   it('should have name "Regex"', () => {
+    const sut = new RegexMutator();
     expect(sut.name).eq('Regex');
   });
 
   it('should not mutate normal string literals', () => {
+    const sut = new RegexMutator();
     expectJSMutation(sut, '""');
   });
 
   it('should mutate a regex literal', () => {
+    const sut = new RegexMutator();
     expectJSMutation(sut, '/\\d{4}/', '/\\d/', '/\\D{4}/');
   });
 
   it("should not crash if a regex couldn't be parsed", () => {
+    // Arrange
+    const weaponRegexStub = sinon.stub();
+    weaponRegexStub.throws(new Error('[Error] Parser: Position 1:1, found "[[]]"'));
+    const sut = new RegexMutator(weaponRegexStub);
     const errorStub = sinon.stub(console, 'error');
+
+    // Act
     expectJSMutation(sut, '/[[]]/');
+
+    // Assert
     expect(errorStub).calledWith(
       '[RegexMutator]: The Regex parser of weapon-regex couldn\'t parse this regex pattern: "[[]]". Please report this issue at https://github.com/stryker-mutator/weapon-regex/issues. Inner error: [Error] Parser: Position 1:1, found "[[]]"'
     );
   });
 
   it('should mutate obvious Regex string literals', () => {
+    const sut = new RegexMutator();
     expectJSMutation(sut, 'new RegExp("\\\\d{4}")', 'new RegExp("\\\\d")', 'new RegExp("\\\\D{4}")');
   });
 
   it('should not mutate the flags of a new RegExp constructor', () => {
+    const sut = new RegexMutator();
     expectJSMutation(sut, 'new RegExp("", "\\\\d{4}")');
   });
 });
diff --git a/packages/instrumenter/tsconfig.src.json b/packages/instrumenter/tsconfig.src.json
index 0f332194d8..63eb139925 100644
--- a/packages/instrumenter/tsconfig.src.json
+++ b/packages/instrumenter/tsconfig.src.json
@@ -15,5 +15,5 @@
       "path": "../util/tsconfig.src.json"
     }
   ],
-  "include": ["src"]
+  "include": ["src", "typings"]
 }
diff --git a/packages/instrumenter/tsconfig.test.json b/packages/instrumenter/tsconfig.test.json
index 5cf5c49895..f86f43821d 100644
--- a/packages/instrumenter/tsconfig.test.json
+++ b/packages/instrumenter/tsconfig.test.json
@@ -16,7 +16,5 @@
       "path": "../test-helpers/tsconfig.src.json"
     }
   ],
-  "include": [
-    "test"
-  ]
+  "include": ["test", "typings"]
 }
diff --git a/packages/instrumenter/src/typings/weapon-regex.ts b/packages/instrumenter/typings/weapon-regex.d.ts
similarity index 100%
rename from packages/instrumenter/src/typings/weapon-regex.ts
rename to packages/instrumenter/typings/weapon-regex.d.ts