Regular expression to match ASCII and Unicode letters

Recently I discovered, to my surprise, that JavaScript has no built-in support for Unicode re开发者_运维问答gular expressions.

So how can I test a string for letters only, Unicode or ASCII?

I'd recommend Steven Levithan's excellent XRegExp library, which has a Unicode plugin containing various Unicode character classes: http://xregexp.com/plugins/

Recently I discovered, to my surprise, that javascript has no builtin support for unicode regex.

This comes to a surprise to me as well because

alert(/\u00B6/.test("¶"));

prints true.