It finds a specified piece of text within your input and replaces every occurrence with your chosen replacement text.
Yes, checking "Use regex" lets you enter a regular expression pattern in the Find field instead of plain literal text.
Your Find text is treated as literal text, with any special characters automatically escaped so they are matched exactly rather than interpreted as regex syntax.
When checked, the search distinguishes between uppercase and lowercase letters; when unchecked, it matches regardless of letter case.
The tool shows a clear error message explaining the regex is invalid, instead of silently failing or breaking the page.