Add warning for unsupported delimiter characters

Fixes #84

src/org/stringtemplate/v4/STGroup.java

@@ -80,6 +80,25 @@ public class STGroup {
 		TEMPLATE_FILE_EXTENSION = ".st";
 	}
 
+	private static final boolean[] RESERVED_CHARACTERS = new boolean[127];
+	static {
+		for (char c = 'a'; c <= 'z'; c++) {
+			RESERVED_CHARACTERS[c] = true;
+		}
+
+		for (char c = 'A'; c <= 'Z'; c++) {
+			RESERVED_CHARACTERS[c] = true;
+		}
+
+		for (char c = '0'; c <= '9'; c++) {
+			RESERVED_CHARACTERS[c] = true;
+		}
+
+		RESERVED_CHARACTERS['@'] = true;
+		RESERVED_CHARACTERS['-'] = true;
+		RESERVED_CHARACTERS['_'] = true;
+	}
+
     /** When we use key as a value in a dictionary, this is how we signify. */
     public static final String DICT_KEY = "key";
     public static final String DEFAULT_KEY = "default";
@@ -293,6 +312,22 @@ public synchronized void unload() {
     /** Force a load if it makes sense for the group. */
     public void load() { }
 
+	/**
+	 * Determines if a specified character may be used as a user-specified delimiter.
+	 *
+	 * @param c The character
+	 * @return {@code true} if the character is reserved by the StringTemplate
+	 * language; otherwise, {@code false} if the character may be used as a
+	 * delimiter.
+	 *
+	 * @since 4.0.9
+	 */
+	public static boolean isReservedCharacter(char c) {
+		return c >= 0
+			&& c < RESERVED_CHARACTERS.length
+			&& RESERVED_CHARACTERS[c];
+	}
+
     protected CompiledST lookupImportedTemplate(String name) {
         if ( imports.size()==0 ) return null;
         for (STGroup g : imports) {

src/org/stringtemplate/v4/compiler/Group.g

@@ -190,8 +190,23 @@ groupName returns [String name]
 delimiters
     :	'delimiters' a=STRING ',' b=STRING
      	{
-     	group.delimiterStartChar=$a.getText().charAt(1);
-        group.delimiterStopChar=$b.getText().charAt(1);
+		boolean supported = true;
+		char startCharacter = $a.getText().charAt(1);
+		if (STGroup.isReservedCharacter(startCharacter)) {
+			group.errMgr.compileTimeError(ErrorType.UNSUPPORTED_DELIMITER, null, $a, String.valueOf(startCharacter));
+			supported = false;
+		}
+
+		char stopCharacter = $b.getText().charAt(1);
+		if (STGroup.isReservedCharacter(stopCharacter)) {
+			group.errMgr.compileTimeError(ErrorType.UNSUPPORTED_DELIMITER, null, $b, String.valueOf(stopCharacter));
+			supported = false;
+		}
+
+		if (supported) {
+			group.delimiterStartChar=$a.getText().charAt(1);
+			group.delimiterStopChar=$b.getText().charAt(1);
+		}
         }
     ;
 

src/org/stringtemplate/v4/misc/ErrorType.java

@@ -59,6 +59,7 @@ public enum ErrorType {
 	INVALID_TEMPLATE_NAME("invalid template name or path: %s"),
 	ANON_ARGUMENT_MISMATCH("anonymous template has %s arg(s) but mapped across %s value(s)"),
 	REQUIRED_PARAMETER_AFTER_OPTIONAL("required parameters (%s) must appear before optional parameters"),
+	UNSUPPORTED_DELIMITER("unsupported delimiter character: %s"),
 
     // INTERNAL ERRORS
     INTERNAL_ERROR("%s"),

test/org/stringtemplate/v4/test/TestGroupSyntax.java

@@ -119,6 +119,30 @@ public class TestGroupSyntax extends BaseTest {
 		assertEquals(expected, result);
 	}
 
+	/**
+	 * This is a regression test for antlr/stringtemplate4#84.
+	 */
+	@Test public void testSetUnsupportedDelimiters_At() throws Exception {
+		String templates =
+			"delimiters \"@\", \"@\"" + Misc.newline +
+			"ta(x) ::= \"[<x>]\"" + Misc.newline;
+
+		writeFile(tmpdir, "t.stg", templates);
+		ErrorBuffer errors = new ErrorBuffer();
+		STGroup group = new STGroupFile(tmpdir+"/"+"t.stg");
+		group.setListener(errors);
+		ST st = group.getInstanceOf("ta");
+		st.add("x", "hi");
+		String expected = "[hi]";
+		String result = st.render();
+		assertEquals(expected, result);
+
+		String expectedErrors = "[t.stg 1:11: unsupported delimiter character: @, "
+			+ "t.stg 1:16: unsupported delimiter character: @]";
+		String resultErrors = errors.errors.toString();
+		assertEquals(expectedErrors, resultErrors);
+	}
+
     @Test public void testSingleTemplateWithArgs() throws Exception {
         String templates =
             "t(a,b) ::= \"[<a>]\"" + Misc.newline;