How to find list of possible words from a letter matrix [Boggle Solver]

My answer works like the others here, but I’ll post it because it looks a bit faster than the other Python solutions, from setting up the dictionary faster. (I checked this against John Fouhy’s solution.) After setup, the time to solve is down in the noise.

grid = "fxie amlo ewbx astu".split()
nrows, ncols = len(grid), len(grid[0])

# A dictionary word that could be a solution must use only the grid's
# letters and have length >= 3. (With a case-insensitive match.)
import re
alphabet="".join(set(''.join(grid)))
bogglable = re.compile('[' + alphabet + ']{3,}$', re.I).match

words = set(word.rstrip('\n') for word in open('words') if bogglable(word))
prefixes = set(word[:i] for word in words
               for i in range(2, len(word)+1))

def solve():
    for y, row in enumerate(grid):
        for x, letter in enumerate(row):
            for result in extending(letter, ((x, y),)):
                yield result

def extending(prefix, path):
    if prefix in words:
        yield (prefix, path)
    for (nx, ny) in neighbors(path[-1]):
        if (nx, ny) not in path:
            prefix1 = prefix + grid[ny][nx]
            if prefix1 in prefixes:
                for result in extending(prefix1, path + ((nx, ny),)):
                    yield result

def neighbors((x, y)):
    for nx in range(max(0, x-1), min(x+2, ncols)):
        for ny in range(max(0, y-1), min(y+2, nrows)):
            yield (nx, ny)

Sample usage:

# Print a maximal-length word and its path:
print max(solve(), key=lambda (word, path): len(word))

Edit: Filter out words less than 3 letters long.

Edit 2: I was curious why Kent Fredric’s Perl solution was faster; it turns out to use regular-expression matching instead of a set of characters. Doing the same in Python about doubles the speed.