diff --git a/xmlregexp.c b/xmlregexp.c
index f1366fd4..f5afa45d 100644
--- a/xmlregexp.c
+++ b/xmlregexp.c
@@ -4907,6 +4907,47 @@ xmlFAParseCharProp(xmlRegParserCtxtPtr ctxt) {
     }
 }
 
+static int parse_escaped_codeunit(xmlRegParserCtxtPtr ctxt)
+{
+    int val = 0, i, cur;
+    for (i = 0; i < 4; i++) {
+	NEXT;
+	val *= 16;
+	cur = CUR;
+	if (cur >= '0' && cur <= '9') {
+	    val += cur - '0';
+	} else if (cur >= 'A' && cur <= 'F') {
+	    val += cur - 'A' + 10;
+	} else if (cur >= 'a' && cur <= 'f') {
+	    val += cur - 'a' + 10;
+	} else {
+	    ERROR("Expecting hex digit");
+	    return -1;
+	}
+    }
+    return val;
+}
+
+static int parse_escaped_codepoint(xmlRegParserCtxtPtr ctxt)
+{
+    int val = parse_escaped_codeunit(ctxt);
+    if (0xD800 <= val && val <= 0xDBFF) {
+	NEXT;
+	if (CUR == '\\') {
+	    NEXT;
+	    if (CUR == 'u') {
+		int low = parse_escaped_codeunit(ctxt);
+		if (0xDC00 <= low && low <= 0xDFFF) {
+		    return (val - 0xD800) * 0x400 + (low - 0xDC00) + 0x10000;
+		}
+	    }
+	}
+	ERROR("Invalid low surrogate pair code unit");
+	val = -1;
+    }
+    return val;
+}
+
 /**
  * xmlFAParseCharClassEsc:
  * @ctxt:  a regexp parser context
@@ -4969,10 +5010,25 @@ xmlFAParseCharClassEsc(xmlRegParserCtxtPtr ctxt) {
 	(cur == '|') || (cur == '.') || (cur == '?') || (cur == '*') ||
 	(cur == '+') || (cur == '(') || (cur == ')') || (cur == '{') ||
 	(cur == '}') || (cur == 0x2D) || (cur == 0x5B) || (cur == 0x5D) ||
-	(cur == 0x5E) || (cur == '!') || (cur == '"') || (cur == '#') ||
-	(cur == '$') || (cur == '%') || (cur == ',') || (cur == '/') ||
-	(cur == ':') || (cur == ';') || (cur == '=') || (cur == '>') ||
-	(cur == '@') || (cur == '`') || (cur == '~') || (cur == 'u')) {
+	(cur == 0x5E) ||
+
+	/* Non-standard escape sequences:
+	 *                  Java 1.8|.NET Core 3.1|MSXML 6 */
+	(cur == '!') ||     /*   +  |     +       |    +   */
+	(cur == '"') ||     /*   +  |     +       |    +   */
+	(cur == '#') ||     /*   +  |     +       |    +   */
+	(cur == '$') ||     /*   +  |     +       |    +   */
+	(cur == '%') ||     /*   +  |     +       |    +   */
+	(cur == ',') ||     /*   +  |     +       |    +   */
+	(cur == '/') ||     /*   +  |     +       |    +   */
+	(cur == ':') ||     /*   +  |     +       |    +   */
+	(cur == ';') ||     /*   +  |     +       |    +   */
+	(cur == '=') ||     /*   +  |     +       |    +   */
+	(cur == '>') ||     /*      |     +       |    +   */
+	(cur == '@') ||     /*   +  |     +       |    +   */
+	(cur == '`') ||     /*   +  |     +       |    +   */
+	(cur == '~') ||     /*   +  |     +       |    +   */
+	(cur == 'u')) {     /*      |     +       |    +   */
 	if (ctxt->atom == NULL) {
 	    ctxt->atom = xmlRegNewAtom(ctxt, XML_REGEXP_CHARVAL);
 	    if (ctxt->atom != NULL) {
@@ -4987,21 +5043,12 @@ xmlFAParseCharClassEsc(xmlRegParserCtxtPtr ctxt) {
 		        ctxt->atom->codepoint = '\t';
 			break;
 		    case 'u':
-		    {
-			char hex_buffer[5];
-			int loop;
-			for (loop = 0; loop < 4; loop++) {
-			    NEXT;
-			    if (!('0' <= CUR && CUR <= '9') && !('a' <= CUR && CUR <= 'f') && !('A' <= CUR && CUR <= 'F')) {
-				ERROR("Expecting hex digit");
-				return;
-			    }
-			    hex_buffer[loop] = CUR;
+			cur = parse_escaped_codepoint(ctxt);
+			if (cur < 0) {
+			    return;
 			}
-			hex_buffer[4] = 0;
-			ctxt->atom->codepoint = (int)strtoul(hex_buffer, NULL, 16);
+			ctxt->atom->codepoint = cur;
 			break;
-		    }
 		    default:
 			ctxt->atom->codepoint = cur;
 		}