regexp: add AsciiUtil.isLetter()

RegExpSupport/src/org/intellij/lang/regexp/AsciiUtil.java

@@ -30,6 +30,10 @@ public final class AsciiUtil {
     return c >= 'a' && c <= 'z';
   }
 
+  public static boolean isLetter(char c) {
+    return isUpperCase(c) || isLowerCase(c);
+  }
+
   public static boolean isDigit(char c) {
     return c >= '0' && c <= '9';
   }

RegExpSupport/src/org/intellij/lang/regexp/RegExpParser.java

@@ -478,7 +478,7 @@ public class RegExpParser implements PsiParser {
   private static boolean isLetter(CharSequence text) {
     assert text.length() == 1;
     final char c = text.charAt(0);
-    return AsciiUtil.isUpperCase(c) || AsciiUtil.isLowerCase(c);
+    return AsciiUtil.isLetter(c);
   }
 
   private void parseProperty(PsiBuilder builder) {