6817475: named-capturing group name started with digit causes PSE exception

Summary: Need accept the digit as the first char of the group name
Reviewed-by: alanb

src/share/classes/java/util/regex/Pattern.java

@@ -2567,7 +2567,8 @@ loop:   for(int x=0, offset=0; x<nCodePoints; x++, offset+=len) {
                 break;
             case '<':   // (?<xxx)  look behind
                 ch = read();
-                if (Character.isLetter(ch)) {     // named captured group
+                if (ASCII.isLower(ch) || ASCII.isUpper(ch) || ASCII.isDigit(ch)) {
+                    // named captured group
                     String name = groupname(ch);
                     if (namedGroups().containsKey(name))
                         throw error("Named capturing group <" + name

test/java/util/regex/RegExTest.java

@@ -3389,6 +3389,11 @@ public class RegExTest {
               "gname",
               "yyy");
 
+        check(Pattern.compile("x+(?<8gname>y+)z+"),
+              "xxxyyyzzz",
+              "8gname",
+              "yyy");
+
         //backref
         Pattern pattern = Pattern.compile("(a*)bc\\1");
         check(pattern, "zzzaabcazzz", true);  // found "abca"