Improve error message for unsupported character in SpEL expression

Prior to this commit, when an unsupported character such as "ü" was
encountered in a SpEL expression, the error message was:

Cannot handle (252) 'ü'

With this commit, the error message is now similar to:

Unsupported character 'ü' (252) encountered at position 5 in expression.

See gh-30580
Closes gh-30602

spring-expression/src/main/java/org/springframework/expression/spel/standard/Tokenizer.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 the original author or authors.
+ * Copyright 2002-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -265,7 +265,9 @@ class Tokenizer {
 						raiseParseException(this.pos, SpelMessage.UNEXPECTED_ESCAPE_CHAR);
 						break;
 					default:
-						throw new IllegalStateException("Cannot handle (" + (int) ch + ") '" + ch + "'");
+						throw new IllegalStateException(
+								"Unsupported character '%s' (%d) encountered at position %d in expression."
+										.formatted(ch, (int) ch, (this.pos + 1)));
 				}
 			}
 		}

spring-expression/src/test/java/org/springframework/expression/spel/ParsingTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -24,6 +24,7 @@ import org.springframework.expression.spel.standard.SpelExpression;
 import org.springframework.expression.spel.standard.SpelExpressionParser;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
 
 /**
  * Parse some expressions and check we get the AST we expect.
@@ -42,6 +43,58 @@ class ParsingTests {
 	@Nested
 	class Miscellaneous {
 
+		@Test
+		void supportedCharactersInIdentifiers() {
+			parseCheck("#var='value'");
+			parseCheck("#Varz='value'");
+			parseCheck("#VarZ='value'");
+			parseCheck("#_var='value'");
+			parseCheck("#$var='value'");
+			parseCheck("#_$_='value'");
+
+			parseCheck("age");
+			parseCheck("getAge()");
+			parseCheck("get$age()");
+			parseCheck("age");
+			parseCheck("Age");
+			parseCheck("__age");
+			parseCheck("get__age()");
+
+			parseCheck("person.age");
+			parseCheck("person.getAge()");
+			parseCheck("person.get$age()");
+			parseCheck("person$1.age");
+			parseCheck("person_1.Age");
+			parseCheck("person_1.__age");
+			parseCheck("Person_1.get__age()");
+		}
+
+		@Test
+		void unsupportedCharactersInIdentifiers() {
+			// Invalid syntax
+			assertThatIllegalStateException()
+					.isThrownBy(() -> parser.parseRaw("apple~banana"))
+					.withMessage("Unsupported character '~' (126) encountered at position 6 in expression.");
+
+			// German characters
+			assertThatIllegalStateException()
+					.isThrownBy(() -> parser.parseRaw("begrüssung"))
+					.withMessage("Unsupported character 'ü' (252) encountered at position 5 in expression.");
+			assertThatIllegalStateException()
+					.isThrownBy(() -> parser.parseRaw("Spaß"))
+					.withMessage("Unsupported character 'ß' (223) encountered at position 4 in expression.");
+
+			// Spanish characters
+			assertThatIllegalStateException()
+					.isThrownBy(() -> parser.parseRaw("buenos_sueños"))
+					.withMessage("Unsupported character 'ñ' (241) encountered at position 11 in expression.");
+
+			// Chinese characters
+			assertThatIllegalStateException()
+					.isThrownBy(() -> parser.parseRaw("have乐趣()"))
+					.withMessage("Unsupported character '乐' (20048) encountered at position 5 in expression.");
+		}
+
 		@Test
 		void literalNull() {
 			parseCheck("null");