Validate regex patterns

semantic/src/main/kotlin/tools/samt/semantic/ConstraintBuilder.kt

@@ -98,9 +98,18 @@ internal class ConstraintBuilder(private val controller: DiagnosticController) {
     private fun createPattern(
         expression: ExpressionNode,
         argument: StringNode,
-    ): ResolvedTypeReference.Constraint.Pattern {
-        // We will validate the pattern here in the future
-        return ResolvedTypeReference.Constraint.Pattern(expression, argument.value)
+    ): ResolvedTypeReference.Constraint.Pattern? {
+        val pattern = argument.value
+
+        try { Regex(pattern) } catch (e: Exception) {
+            argument.reportError(controller) {
+                message("Invalid regex pattern: '${e.message}'")
+                highlight(argument.location)
+            }
+            return null
+        }
+
+        return ResolvedTypeReference.Constraint.Pattern(expression, pattern)
     }
 
     private fun createValue(

semantic/src/test/kotlin/tools/samt/semantic/SemanticModelTest.kt

@@ -334,6 +334,24 @@ class SemanticModelTest {
             )
         }
 
+        @Test
+        fun `pattern must be valid`() {
+            val source = """
+                package complex
+
+                record Foo {
+                    name: String (pattern("fo/+++!hi"))
+                }
+            """.trimIndent()
+            parseAndCheck(
+                source to listOf(
+                    "Error: Invalid regex pattern: 'Dangling meta character '+' near index 5${System.lineSeparator()}" +
+                            "fo/+++!hi${System.lineSeparator()}" +
+                            "     ^'"
+                )
+            )
+        }
+
         @Test
         fun `cannot use non-existent constraints`() {
             val source = """

specification/examples/debug/debug.samt

@@ -0,0 +1,5 @@
+package debug.test
+
+record Foo {
+    name: String ("++")
+}

specification/examples/debug/samt.yaml

@@ -0,0 +1,5 @@
+source: ./
+
+generators:
+  - name: kotlin-ktor-consumer
+    output: ./out