Why regular expression for cyrillic letters misses a letter? [duplicate]

Question

I want to validate a text input field in a html page to accept only Cyrillic letters. I have written the validation code in JavaScript using regular expression like this:

var namevalue = document.getElementById("name")
var letters = /^[А-Яа-я]+$/;
if (namevalue.matches(letters)) {
  alert("Accepted");
}
else {
  alert("Enter only cyrillic letters");
}

This code works fine for all cyrillic letters except Ё ё

Answer 1

The problem why ё is not working because it's out of range Aа-Яя. Aа-Яа is in a Basic Cyrillic alphabet [0430-044F], but ё isn't in that Basic Cyrillic alphabet. ё belongs to Cyrillic extensions [0400-045F]. Because, JavaScript regexs engine compares not by letters itself but by its charcodes, so ё just is out of range.

Since I presume you mean modern Russian language where despite ё is rare but still in wide use I may suggest this solution

var namevalue = document.getElementById("name")

// please note that I added to your pattern "еЁ".
// now this matches all Russian cyrillic letters both small and caps
// plus ё and Ё
var letters = /^[А-Яа-яёЁ]+$/; 

if (namevalue.matches(letters)) {
   alert("Accepted");
}
else {
   alert("Enter only cyrillic letters");
} 

Unfortunately the problem with A-Я and Ё buried deep in Unicode specification. There is no plain and simple solution. So for robust programming you need always be prepared for such cases.