The following table lists optional parameters that add backreference modifiers to a regular expression.
Backreference. For example, (\w)\1 finds doubled word characters.
Named backreference. For example, (?<char>\w)\k<char> finds doubled word characters. The expression (?<43>\w)\43 does the same. You can use single quotes instead of angle brackets; for example, \k'char'.
Note the ambiguity between octal escape codes and \number backreferences that use the same notation. See Backreferences for details on how the regular expression engine resolves the ambiguity.