I did not quite get your question but from what I did, it looks like you are trying to replace code using Find and Replace. Because there have been changes to the Find and Replace functionality in 2017, can you let us know the version of DW that you are currently using? Also, is this issue that you mention seen only in the latest version?
How many characters are you trying to replace?
There is a character limit in the Replace field.
i have the last version Dreamwaver 2017.0.1.
In dreamweaver cc6 i cant put any lines of code, but the last version only no.
This version has a limit of 1232 characters in the Replace field. I filed a bug report below.
Also in the Ideas section.
Thanks for report the bug...