universal character name

A universal character name consists of a sequence of characters that represent a Unicode code point. These take two forms. 
* Use \UNNNNNNNN to represent a Unicode code point of the form U+NNNNNNNN, where NNNNNNNN is the eight-digit hexadecimal code point number. 
* Use four-digit \uNNNN to represent a Unicode code point of the form U+0000NNNN.