Urgent!!! Regular expression??

[psk]

I need a regular expression for replacing each character in a text
with an X.

For example,

ASASasdfdfDFDFDFDFAAAAAAAAAAAAAAAAAAAAAAABNNBMBMBMBMBBNBJFFFJFFHFFJFJFJJFJFJFJFJFJFJFJFWEWEW

is the initial string i want to replace the 12 A's followed by
something with X's

something like AAAAAAAAAAAAAAAA.* to be replaced by an equal number of
X's

so output should be

ASASasdfdfDFDFDFDFXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

Thanx for ur help.

 

[dw]

I need a regular expression for replacing each character in a text
with an X.

For example,

ASASasdfdfDFDFDFDFAAAAAAAAAAAAAAAAAAAAAAABNNBMBMBMBMBBNBJFFFJFFHFFJFJFJJFJFJ
FJFJFJFJFJFWEWEW

is the initial string i want to replace the 12 A's followed by
something with X's

something like AAAAAAAAAAAAAAAA.* to be replaced by an equal number of
X's

so output should be

ASASasdfdfDFDFDFDFXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXX

how about:
$_ =
'ASASasdfdfDFDFDFDFAAAAAAAAAAAAAAAAAAAAAAABNNBMBMBMBMBBNBJFFFJFFHFFJFJFJJFJF
JFJFJFJFJFJFWEWEW';

s/(A{12}.*)/"X" x length $1/e;