Fix decoding of pkl-binary into Java Pattern (#1895)

Patterns that originate from the language runtime will add
`Pattern.UNICODE_CHARACTER_CLASS | Pattern.UNICODE_CASE` as implicit
regex flags, so we need to do the same on the binary decoding side.

pkl-core/src/main/java/org/pkl/core/util/pklbinary/AbstractPklBinaryDecoder.java

@@ -31,6 +31,7 @@ import org.msgpack.core.MessageUnpacker;
 import org.pkl.core.DataSizeUnit;
 import org.pkl.core.DurationUnit;
 import org.pkl.core.Pair;
+import org.pkl.core.runtime.VmUtils;
 import org.pkl.core.util.LateInit;
 
 /**
@@ -317,10 +318,11 @@ public abstract class AbstractPklBinaryDecoder {
     return result;
   }
 
+  @SuppressWarnings("MagicConstant")
   private Object decodeRegex(int len) throws IOException {
     assertLength(PklBinaryCode.REGEX, len, 1);
     currPath.push("'regex");
-    var result = doDecodeRegex(Pattern.compile(unpacker.unpackString()));
+    var result = doDecodeRegex(Pattern.compile(unpacker.unpackString(), VmUtils.REGEX_FLAGS));
     unpacker.skipValue(len - 2);
     currPath.pop();
     return result;

pkl-core/src/test/kotlin/org/pkl/core/PklBinaryDecoderTest.kt

@@ -20,6 +20,7 @@ import java.util.regex.Pattern
 import org.assertj.core.api.Assertions.assertThat
 import org.junit.jupiter.api.Test
 import org.junit.jupiter.api.assertThrows
+import org.msgpack.core.MessagePack
 import org.pkl.core.util.pklbinary.PklBinaryCode
 
 class PklBinaryDecoderTest {
@@ -261,4 +262,14 @@ class PklBinaryDecoderTest {
       "Unable to decode set of length 65536, exceeded maximum collection size",
     )
   }
+
+  @Test
+  fun `decode regex preserves Pkl's implicit flags`() {
+    val packer = MessagePack.newDefaultBufferPacker()
+    packer.packArrayHeader(2)
+    packer.packByte(0x0B)
+    packer.packString("(?i)é")
+    val decoded = PklBinaryDecoder.decode(packer.toByteArray()) as Pattern
+    assertThat(decoded.matcher("É")).matches()
+  }
 }