EDIT: Added Java version at end — despite being inherently clumsy, unreadable, and unmaintainable. ¡No More Ugly Patterns! The first thing you need to do is to write your regex in a way that can stand any possible hope of being readable — and consequently maintainable — by human beings. The second thing you need … Read more
What is the idiomatic approach to this class of problems? Is there a way to simplify the problem? Many of the following remarks could be added to many programs here on SO. Imperative names Every time, you write an imperative name for something that is a relation you will reduce your understanding of relations. Not … Read more
There is a standard algorithm for solving this problem that is based on Gaussian elimination over GF(2). The idea is to set up a matrix representing the button presses a column vector representing the lights and then to use standard matrix simplification techniques to determine which buttons to press. It runs in polynomial time and … Read more
Explanation Nested group doesn’t automatically causes catastrophic backtracking. In your case, it is because your regex degenerates to the classical example of catastrophic backtracking (a*)*. Since \s in optional in ^([a-zA-Z0-9′-]+\s?)*$, on input without any spaces but has characters outside the allowed list, the regex simply degenerates to ^([a-zA-Z0-9′-]+)*$. You can also think in term … Read more
Your current regex can be written as ^(?:[a-zA-Z]:\\|\\\\)([^\\\/\:*?<>”|]+\\?)+$: pay attention at the ? quantifier (it is equal to {0,1} limiting quantifier) after \\ inside a + quantified group. Once such a pattern like (a+b?)+ is present inside a pattern, there is a high chance of a catastrophical backtracking. Everything is nice when there is a … Read more
Generating all the possible permutations and then discarding the repeated ones is highly inefficient. Various algorithms exist to directly generate the permutations of a multiset in lexicographical order or other kind of ordering. Takaoka’s algorithm is a good example, but probably that of Aaron Williams is better http://webhome.csc.uvic.ca/~haron/CoolMulti.pdf moreover, it has been implemented in the … Read more
There is an important observation (in a comment) that a partition of a set of n elements can be represented as an integer sequence of the form [p1, … pn] where pi is the partition number of element i. For such a sequence to be valid, it must obey the rules that p1 is 1, … Read more