What to run
Open Find and Replace. It stays in the browser.
How to read the result
The output is the paste with replacements.
A count of replacements tells you whether the find string was too narrow.
This is literal unless the page exposes regex.
What to open next
Open Regex Tester to design a pattern, then Find and Replace. Both stay in the browser.
Open Text Diff against the original to review. It stays in the browser.