Finding repeating pairs of specific numbers within column

I'm stuck on this probably very trivial problem:

I have a matrix like this (just much larger!):

A = [1 2 4 10 11 12 14 17 19; 5 30 30 50 90 70 5 30 5]'

A =

1 5

2 30 <--

4 30

10 50

11 90

12 70

14 5

17 30 <--

19 5

I'm now looking for "pairs" of specific numbers in A(:, 2). For example, I'd like to find all 30s that are preceded by 5s in A(:, 2) and extract the corresponding row in A(:, 1).

So I'd like to end up, in this example, with B = [2, 17];

How can I do that?

[Phillip]

Vicky from stackoverflow solved this nicely - Thank you!!:

num_wanted=30;

num_previous=5;

is_match=[ false; A(1:end-1,2)==num_previous & A(2:end,2)==num_wanted ]; %the first element cannot be a match

A(is_match,1)