What to run
Open Regex Tester. It stays in the browser.
How to read the result
Matches are what JavaScript found, with groups.
No match is a real result.
Flags change it: i ignores case, m changes anchors, g finds more than one.
A match on a sample is not proof it is safe on untrusted input.
What to open next
Open Find and Replace if the pattern is only for a one-off edit. It stays in the browser.
Open Email Validator for a single address rather than a homemade email regex. It checks syntax only and stays in the browser.