Note that the string-based answers supplied above should work in most cases. The one major advantage offered by a regex solution is that you can more easily provide for a case-insensitive match on the open/close body tags. If that is not a concern to you, then there's no major reason to use regex here.
And for the people who see HTML and regex together and throw a fit...Since you are not actually trying to parse HTML with this, it is something you can do with regular expressions. If, for some reason, content contained </body> then it would fail, but aside from that, you have a sufficiently specific scenario that regular expressions are capable of doing what you want:
const strVal = yourStringValue; //obviously, this line can be omitted - just assign your string to the name strVal or put your string var in the pattern.exec call below
const pattern = /<body[^>]*>((.|[\n\r])*)<\/body>/im;
const array_matches = pattern.exec(strVal);
After the above executes, array_matches[1] will hold whatever came between the <body and </body> tags.
