Add RegEx validation for @pattern decorator (microsoft#5252)

... and throw a warning for invalid regex string value.

close microsoft#4128

.chronus/changes/azhang_ValidatePatternRegEx-2024-11-3-11-32-36.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/compiler"
+---
+
+Added RegEx validation for @pattern and will throw warning for invalid RegEx string

packages/compiler/src/core/messages.ts

@@ -781,6 +781,13 @@ const diagnostics = {
   /**
    * Decorator
    */
+  "invalid-pattern-regex": {
+    severity: "warning",
+    messages: {
+      default: "@pattern decorator expects a valid regular expression pattern.",
+    },
+  },
+
   "decorator-wrong-target": {
     severity: "error",
     messages: {

packages/compiler/src/lib/decorators.ts

@@ -414,6 +414,14 @@ export const $pattern: PatternDecorator = (
     return;
   }
 
+  try {
+    new RegExp(pattern);
+  } catch (e) {
+    reportDiagnostic(context.program, {
+      code: "invalid-pattern-regex",
+      target: target,
+    });
+  }
   const patternData: PatternData = {
     pattern,
     validationMessage,

packages/compiler/test/decorators/decorators.test.ts

@@ -261,6 +261,19 @@ describe("compiler: built-in decorators", () => {
       });
     });
 
+    it("emit diagnostic if pattern is not a valid RegEx", async () => {
+      const diagnostics = await runner.diagnose(`
+        model A {
+          @pattern("[a-z")
+          prop: string;
+        }
+      `);
+
+      expectDiagnostics(diagnostics, {
+        code: "invalid-pattern-regex",
+      });
+    });
+
     it("optionally allows specifying a pattern validation message", async () => {
       const { A, B } = (await runner.compile(
         `