diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/Swift5ClientCodegen.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/Swift5ClientCodegen.java
index 7d2a1d9d5154..0ff51c2bd6b5 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/Swift5ClientCodegen.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/Swift5ClientCodegen.java
@@ -1264,6 +1264,16 @@ public void postProcessModelProperty(CodegenModel model, CodegenProperty propert
         }
     }
 
+    @Override
+    public String toRegularExpression(String pattern) {
+        // Don't wrap the pattern in "/.../" delimiters: the generated
+        // Validator hands rule.pattern straight to NSRegularExpression, which
+        // has no delimiter syntax. Wrapping also escaped every inner "/" as
+        // "\/", which is not a valid escape sequence in a Swift string
+        // literal, so any pattern containing "/" failed to compile (#15604).
+        return escapeText(pattern);
+    }
+
     @Override
     public String escapeQuotationMark(String input) {
         // remove " to avoid code injection
diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/Swift6ClientCodegen.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/Swift6ClientCodegen.java
index 7cd975547f2c..8c7370751f10 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/Swift6ClientCodegen.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/Swift6ClientCodegen.java
@@ -1321,6 +1321,16 @@ public void postProcessModelProperty(CodegenModel model, CodegenProperty propert
         }
     }
 
+    @Override
+    public String toRegularExpression(String pattern) {
+        // Don't wrap the pattern in "/.../" delimiters: the generated
+        // Validator hands rule.pattern straight to NSRegularExpression, which
+        // has no delimiter syntax. Wrapping also escaped every inner "/" as
+        // "\/", which is not a valid escape sequence in a Swift string
+        // literal, so any pattern containing "/" failed to compile (#15604).
+        return escapeText(pattern);
+    }
+
     @Override
     public String escapeQuotationMark(String input) {
         // remove " to avoid code injection
diff --git a/modules/openapi-generator/src/test/java/org/openapitools/codegen/swift5/Swift5ClientCodegenTest.java b/modules/openapi-generator/src/test/java/org/openapitools/codegen/swift5/Swift5ClientCodegenTest.java
index ec64a91269d1..2723cad4e698 100644
--- a/modules/openapi-generator/src/test/java/org/openapitools/codegen/swift5/Swift5ClientCodegenTest.java
+++ b/modules/openapi-generator/src/test/java/org/openapitools/codegen/swift5/Swift5ClientCodegenTest.java
@@ -35,6 +35,22 @@ public class Swift5ClientCodegenTest {
 
     Swift5ClientCodegen swiftCodegen = new Swift5ClientCodegen();
 
+    @Test(enabled = true)
+    public void testToRegularExpressionRemainsValidInSwiftStringLiteral() throws Exception {
+        // patterns are passed verbatim to NSRegularExpression at runtime, so no
+        // "/.../" delimiters are added and, in particular, no "\/" escape is
+        // produced ("\/" is not a valid escape sequence in a Swift string
+        // literal, see issue #15604)
+        Assert.assertEquals(swiftCodegen.toRegularExpression("http(s)?://x"), "http(s)?://x");
+        Assert.assertEquals(swiftCodegen.toRegularExpression("[a-z/]+"), "[a-z/]+");
+        // "\/" in the spec (a JSON-style escaped slash) is normalized to "/"
+        Assert.assertEquals(swiftCodegen.toRegularExpression("http(s)?:\\/\\/x"), "http(s)?://x");
+        // backslashes are escaped for the Swift string literal
+        Assert.assertEquals(swiftCodegen.toRegularExpression("[a-z0-9\\-]+\\.[a-z]{2,63}"), "[a-z0-9\\\\-]+\\\\.[a-z]{2,63}");
+        // a pattern that already carries delimiters is left untouched
+        Assert.assertEquals(swiftCodegen.toRegularExpression("/[a-z]/i"), "/[a-z]/i");
+    }
+
     @Test(enabled = true)
     public void testCapitalizedReservedWord() throws Exception {
         Assert.assertEquals(swiftCodegen.toEnumVarName("AS", null), "_as");
diff --git a/modules/openapi-generator/src/test/java/org/openapitools/codegen/swift6/Swift6ClientCodegenTest.java b/modules/openapi-generator/src/test/java/org/openapitools/codegen/swift6/Swift6ClientCodegenTest.java
index dfd3cd8796ac..4d776f9b2ac6 100644
--- a/modules/openapi-generator/src/test/java/org/openapitools/codegen/swift6/Swift6ClientCodegenTest.java
+++ b/modules/openapi-generator/src/test/java/org/openapitools/codegen/swift6/Swift6ClientCodegenTest.java
@@ -36,6 +36,22 @@ public class Swift6ClientCodegenTest {
 
     Swift6ClientCodegen swiftCodegen = new Swift6ClientCodegen();
 
+    @Test(enabled = true)
+    public void testToRegularExpressionRemainsValidInSwiftStringLiteral() throws Exception {
+        // patterns are passed verbatim to NSRegularExpression at runtime, so no
+        // "/.../" delimiters are added and, in particular, no "\/" escape is
+        // produced ("\/" is not a valid escape sequence in a Swift string
+        // literal, see issue #15604)
+        Assert.assertEquals(swiftCodegen.toRegularExpression("http(s)?://x"), "http(s)?://x");
+        Assert.assertEquals(swiftCodegen.toRegularExpression("[a-z/]+"), "[a-z/]+");
+        // "\/" in the spec (a JSON-style escaped slash) is normalized to "/"
+        Assert.assertEquals(swiftCodegen.toRegularExpression("http(s)?:\\/\\/x"), "http(s)?://x");
+        // backslashes are escaped for the Swift string literal
+        Assert.assertEquals(swiftCodegen.toRegularExpression("[a-z0-9\\-]+\\.[a-z]{2,63}"), "[a-z0-9\\\\-]+\\\\.[a-z]{2,63}");
+        // a pattern that already carries delimiters is left untouched
+        Assert.assertEquals(swiftCodegen.toRegularExpression("/[a-z]/i"), "/[a-z]/i");
+    }
+
     @Test(enabled = true)
     public void testCapitalizedReservedWord() throws Exception {
         Assert.assertEquals(swiftCodegen.toEnumVarName("AS", null), "_as");