Pattern find and replace lets you find certain structures in your code and then modify them. It does not use a textual search, but works on the actual code structure itself.
For example, you could change your code so that things which looked like
If[ x, y = expr]
got changed to look like
If[ x, y = {expr^2}]
You would open the Source popup menu in the Mathematica
source editor by right-clicking and choosing Source >
Expression Find/Replace. You would then fill in the
dialog as follows.
When you click Preview this allows you to preview all the changes that were introduced. From this you can accept or reject the changes. When they are accepted the changes are made.
Before the transformation some of the code might look as follows.
After the transformation it has changed to the following.
Note that as the transformation was carried out, a comment in the code was preserved, as well as the original formatting. This technology is a powerful way to make modifications to your source code.