[Solved]: Are regex crosswords NP-hard?

Problem Detail: 

I was fooling around the other day on this website: http://regexcrossword.com/ and it got me wondering what the best way to solve it was.

Can you solve the following problem in polynomial time or is it NP-hard?

Given an NxM grid with N regular expressions for the columns and M for the rows, find any solution to the grid such that all the regular expressions are satisfied, or say that no solution exists.

Asked By : Glen Takahashi

Answered By : FrankW

The problem is NP-hard.

We show this by reducing vertex cover:

Given a graph $G=(V,E)$ and a threshold $k$, is there a subset $V' \subseteq V$ of cardinality at most $k$, so that each edge in $E$ is incident to at least one node in $V'$?

We translate this into a regex crossword with $|E|+1$ columns and $|V|$ rows as follows:

All columns, except for the first, correspond to an edge. They get a regex $0^*1(0|1)^*$.

All rows correspond to a vertex. They get a regex that allows to write either

• a $1$ in the first column and each column corresponding to an edge incident to that node and zeroes in all other columns, or

• $0^*$

Finally, the first column counts the size of the vertex cover. It gets a regex, that allows for at most $k$ ones.

The correspondence between solutions to the regex crossword and vertex covers should be obvious.

Example:

Find a vertex cover of size 2 for the following graph:

$V_A = 0^* \big| 10110$

$V_B = 0^* \big| 11101$

$V_C = 0^* \big| 10011$

$V_D = 0^* \big| 11000$

$Counter = 0^* \big| 0^*10^* \big| 0^*10^*10^*$

$E_1 = 0^*1(0|1)^*$

$E_2 = 0^*1(0|1)^*$

$E_3 = 0^*1(0|1)^*$

$E_4 = 0^*1(0|1)^*$

Lastly, you can set up the "crossword" so that $V_A$ through $V_D$ are the top regexes and $Counter$ and $E_1$ through $E_4$ are the left side regexes.

Solving this regex crossword give you a vertex cover of size 2 for nodes $V_A,V_B$ or $V_C,V_B$.

If we change k to be 1 and $Counter$ to be $0^* \big| 0^*10^*$ as another example, the regex crossword is impossible to solve because there is no vertex cover of size 1.

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Question Source : http://cs.stackexchange.com/questions/30143