[bugfix] Require ! flag for empty-matching regex in XQ4 fn:replace and fn:tokenize

Without the ! flag, empty-matching patterns raise FORX0003 in both XQ 3.1
and XQ 4.0 mode. With the ! flag in XQ 4.0, fn:replace uses the Java regex
fallback and fn:tokenize tokenizes between each character.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: joewiz

exist-core/src/main/java/org/exist/xquery/functions/fn/FunReplace.java

@@ -151,18 +151,19 @@ public Sequence eval(final Sequence[] args, final Sequence contextSequence) thro
 			try {
 				final RegularExpression regularExpression = config.compileRegularExpression(pattern, flags, "XP30", warnings);
 				final boolean canMatchEmpty = regularExpression.matches("");
+				final boolean allowEmptyMatch = flags.contains("!");
 
 				// XQ 3.1: FORX0003 if regex can match empty string
-				// XQ 4.0: empty-matching regex is allowed
-				if (canMatchEmpty && context.getXQueryVersion() < 40 && !isFunctionReplacement) {
+				// XQ 4.0: empty-matching regex allowed only with the ! flag
+				if (canMatchEmpty && (context.getXQueryVersion() < 40 || !allowEmptyMatch) && !isFunctionReplacement) {
 					throw new XPathException(this, ErrorCodes.FORX0003, "regular expression could match empty string");
 				}
 
 				if (isFunctionReplacement) {
 					result = evalFunctionReplacement(string, pattern, flags,
 							(FunctionReference) replacementArg.itemAt(0));
-				} else if (canMatchEmpty) {
-					// XQ4: empty-matching regex allowed — use Java regex fallback
+				} else if (canMatchEmpty && allowEmptyMatch) {
+					// XQ4: empty-matching regex allowed with ! flag — use Java regex fallback
 					// since Saxon's replace() doesn't handle empty matches well
 					result = evalEmptyMatchReplace(string, pattern, replace, flags);
 				} else {

exist-core/src/main/java/org/exist/xquery/functions/fn/FunTokenize.java

@@ -128,13 +128,13 @@ public Sequence eval(final Sequence[] args, final Sequence contextSequence) thro
                         final Pattern pat = PatternFactory.getInstance().getPattern(pattern, flags);
 
                         if (pat.matcher("").matches()) {
-                            // XQ 3.1: FORX0003 if regex can match empty string
-                            // XQ 4.0: empty-matching regex is allowed
-                            if (context.getXQueryVersion() < 40) {
+                            // XQ4 with ! flag: empty-matching allowed — tokenize between each character
+                            // XQ 3.1 or XQ4 without ! flag: FORX0003
+                            if (context.getXQueryVersion() >= 40 && flagsStr.contains("!")) {
+                                result = tokenizeEmptyMatch(string, pat);
+                            } else {
                                 throw new XPathException(this, ErrorCodes.FORX0003, "regular expression could match empty string");
                             }
-                            // XQ4: empty-matching regex allowed — tokenize between each character
-                            result = tokenizeEmptyMatch(string, pat);
                         } else {
                             final String[] tokens = pat.split(string, -1);
                             result = new ValueSequence();