Hi Guys/Gals

I got a quick question on regex. My input string consists of two
characters: [a-z][a-z].. However, I only want to match if the two
characters are different.

Example: ab,ck,dk,gk... are all valid, but aa,bb,cc,dd,ee... should not
match..

This is the regex I have so far, but it does not seem to work:

([a-z])(?!\1)[a-z]

complete program:
Pattern pat2 = Pattern.compile("([a-z])(?!\1)[a-z]");
Matcher m = pat2.matcher("aa");
System.out.println(m.matches()); THIS PRINTS TRUE but aa should not be
true.

Thanks in Advance,
Saad.