As mentioned in the comments it's a bit rude to ask others to do your job, when you haven't even tried yourself.

The following pattern should work:

Pattern:

_[a-z]+\.[0-9]+

Replace with:

_inf$0

Explanation:

"_" matches the underscore literal so we find the start of the pattern we're looking for

"[a-z]+" matches any lowercase letter any number of times (there is no straightforward way to match exactly 3 letters)

"\." matches the dot literal (escaped by a backslash because the dot is a regex expression itself)

"[0-9]+" matches any digit any number of times (so it can occur 3 or 4 times and still gets matched)

_inf$0 replaces the matched pattern with "_inf" plus the matched pattern itself