{\N} - backreference to an N-th group, the same as a simple \N
Except syntax, named backreferences behaves the same way as usual backreferences.
Example:
regexes "({Letter}\w){\Letter}" and "({1}\w){\1}" match double word chars (namely "aa" and "cc") in a string "123aabccd";

E* - matches a serie of 0 or more of element E, first trying to match as large repetition number as possible (the 'greedy' behaviour);

E+ - matches a serie of 1 or more of element E, first trying to match as large repetition number as possible;

E? - matches 1 or 0 of E, first trying 1;

E{n} - matches a serie of exactly n of element E;

E{m,n} - matches a serie of m to n of element E, first trying to match as large repetition number as possible;

E{,n} - matches a serie of 0 to n of element E, first trying to match as large repetition number as possible;

E{m,} - matches a serie of m or more of element E, first trying to match as large repetition number as possible;

E*? , E+? , E?? , E{n}? , E{m,n}? , E{,n}? , E{m,}? - the '?' after a quantifier changes its behaviour: now it first matches a least possible repetition number;
Note that if a pattern allows E to be zero-width
(such as "\b" or "(:?abc|)"),
then any of E* , E+ , E{m,} may cause an infinite loop.