How do I do complex pattern matching in a source code?

If I have a source code, how do I check if the source code contains some code patterns? For example, if you exchange two variables'value in Java, you wanna do

int a = 1;
int b = 2;
int temp = a;
a = b;
b = temp;

How do I know if a source code will do the swapping? Does anyone know any useful tools to check that? I know some tools to find expression pattern matching, but this is not an expression matching.