By default, findRE finds all matches. You can limit the number of matches with an optional LIMIT argument.
This example returns a slice of all second level headings (h2 elements) within the rendered .Content:
{{ findRE `(?s)<h2.*?>.*?</h2>` .Content }}
The s flag causes . to match \n as well, allowing us to find an h2 element that contains newlines.
To limit the number of matches to one:
{{ findRE `(?s)<h2.*?>.*?</h2>` .Content 1 }}