New to Java

In brief, here is my problem. I'm attempting to scan a string for a very large number that will always start after a ";" and end just before a "=" (without the quotes). I thought I might be able to do this with useDelimiter but it seems I'm on the wrong path here. Could anyone point me toward what I need to grab a string from within two certain points.

For reference, the string will look like this:

;################=xxxxxxxxxxxxxxxx?

Where # represents some number


P.S. I cannot use substring because the number in question is of variable length.