These optional parameters add backreference modifiers to a regular expression.
\number | Backreference. For example, (\w)\1 finds doubled word characters. |
\k<name> | Named backreference. Example: (?<char>\w)\k<char> finds doubled word characters. The expression (?<43>\w)\43 does the same. Single quotes can be used as an alternative delimiter to the angles; for example, \k'char'. |
Note the ambiguity between \number backreferences and octal codes. For more information see Backreferences.