Quick regex_search/replace, or clear indication of replacement?

I must browse a collection of strings to replace a pattern and save the changes.
The saving operation is (very) expensive and out of my hands, so I would like to know beforehand if the replacement did anything.

I can use std::regex_search to gain knowledge on the pattern's presence in my input, and use capture groups to store details in a std::smatch. std::regex_replace does not seem to explicitely tell me wether it did anything.

The patterns and strings are arbitrarily long and complicated; running regex_replace after a regex_search seems wasteful.
I can directly compare the input and output to search for a discrepancy but that too is uncomfortable.

Is there either a simple way to observe regex_replace to determine its impact, or to use a smatch filled by the regex_search to do a faster replacement operation ?

Thanks in advance.