Small confusion about negative lookbehind

I'm writing a small test program to illustrate several aspects of
regular expressions. In the section illustrating "lookaround"s, I
found something I didn't understand. My testing is with JDK 1.4.2.

My candidate string is "ab".

The expressions I'm testing this string against are the following,
which also lists whether the string matched or not

a(?=b) // succeeds
(?=a)b // fails
(?<=a)b // succeeds
a(?<=b) // fails
(?<!x)b // succeeds
a(?<!x) // succeeds(!)

Looking at these, I first wonder what exactly is the semantic
difference between a "lookbehind" and "lookahead" construct. The
syntactic difference is obvious, but I find the question of why pattern
1 succeeds and pattern 2 fails is a little hazy. The one that really
bothers me, however, is pattern 6. Despite the lack of clarity I have
in how this is supposed to work, I was pretty certain that this pattern
would fail.

I could use some clarification of these constructs.

.

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