None of these examples are for parsing English sentences. They parse completely different formal languages. That it’s text is irrelevant, regex usually operates on text.
You cannot write a regex to give you for example “the subject of an English sentence”, just as you can’t write a regex to give you “the contents of a complete div tag”, because neither of those are regular languages (HTML is context-free, not sure about English, my guess is it would be considered recursively enumerable).
You can’t even write a regex to just consume <div> repeated exactly n times followed by </div> repeated exactly n times, because that is already a context-free language instead of a regular language, in fact it is the classic example for a minimal context-free language that Wikipedia also uses.
None of these examples are for parsing English sentences. They parse completely different formal languages. That it’s text is irrelevant, regex usually operates on text.
You cannot write a regex to give you for example “the subject of an English sentence”, just as you can’t write a regex to give you “the contents of a complete div tag”, because neither of those are regular languages (HTML is context-free, not sure about English, my guess is it would be considered recursively enumerable).
You can’t even write a regex to just consume
<div>repeated exactly n times followed by</div>repeated exactly n times, because that is already a context-free language instead of a regular language, in fact it is the classic example for a minimal context-free language that Wikipedia also uses.