Python, connecting vertical lists

Question

I have this for example this characters:

.....U......
L...L.######
.S....#.....
....L.......

which i stored in a list

chars = ['.....U......', 'L...L.######', '.S....#.....', '....L.......']

I used this to store it in the chars:

for x in range(0, N):
    g = input()
    chars.append(g)

Now the problem is i want to turn all of the dots between letters L into # but vertically, so like this:

.....U......
L...L.######
.S..#.#.....
....L.......

I have been trying for a few hours now and i can't think of anything. Help much appreciated.

EDIT: I used this to connect them horizontally. And it works.

while y != N:
    modchars0 = list(chars[y])

    if modchars0.count('L') == 0:
        y += 1

    else:
        for k in range(0, M):
            if 'L' in modchars0[k]:
                start = k + 1
                break

        for l in range(M-1, 0, -1):
            if 'L' in modchars0[l]:
                end = l
                break

        for h in range(start, end):
            if 'L' in modchars0[h]:
                pass
            else:
                modchars0[h] = '#'

        modchars1 = modchars1.join(modchars0)
        chars[y] = modchars1
        y += 1

Answer 1

As @U2EF1 mentioned in comments you can take the transpose of the list using zip(*chars) and then using regex you can convert dots between 'L' to '#'. And then in the end zip(*) the new items again to get desired output:

>>> import re                                                             
>>> r = re.compile(r'(?<=L).*(?=L)')                     
>>> def rep(m):                                           
    return m.group().replace('.', '#')
... 
>>> zipped = (r.sub(rep, ''.join(x)) for x in zip(*chars))
>>> for x in zip(*zipped):
    print ''.join(x)
...     
.....U......
L...L.######
.S..#.#.....
....L.......

Answer 2

Late Answer

This one probably doesn't deserve best answer, but is an alternative nevertheless.

First define a function that will rotate the list from horizontal to vertical:

def invert(chars):
    inverted = []
    for i in range(len(chars[0])):
        newStr=""
        for j in range(len(chars)):
            newStr+=chars[j][i]
        inverted.append(newStr)
    return inverted

Then you may use it in this manner:

def main():
    chars = ['.....U......', 'L...L.######', '.S....#.....', '....L.......']
    invChars = invert(chars)

    for i in range(len(invChars)):
        invChars[i] = re.sub(r'(?<=L)(\..*?)L', lambda k: k.group().replace('.','#'), invChars[i])

    chars = invert(invChars)