What regular expression is needed to replace >piece< of the class/id name?

Question

HTML

I'm trying to replace part of the class/id name with regular expressions, but all attempts result in the entire quoted string being highlighted instead of the specific part of the name.
There are many names, but they all have the same part of the name.
For example, there is with a part of the name old

="old-item-name input_old_category input_old_style"

replace with new part new

="new-item-name input_new_category input_new_style"

the position of the part of the class name / id can be different class-old-class class_old_class old-class old_class An
expression is needed that would find the word old to replace in the class name without affecting the rest of the class name, and would search between the symbols equals and brackets = "something class name"
Something like =".*old.*" but that would highlight not the entire line, but specifically the word old