php - Why use dom to parse webpages instead of regex?

A DOM parser is actually parsing the page.

A regular expression is searching for text, not understanding the HTML's semantic meaning.

It is provable that HTML is not a regular language; therefore, it is impossible to create a regular expression that will parse all instances of an arbitrary element-pattern from an HTML document without also matching some text which is not an instance of that element-pattern.

You may be able to design a regular expression which works for your particular use case, but foreseeing exactly the HTML with which you'll be provided (and, consequently, how it will break your limited-use-case regex) is extremely difficult.

Additionally, a regex is harder to adapt to changes in a page's contents than an XPath expression, and the XPath is (in my mind) easier to read, as it need not be concerned with syntactic odds and ends like tag openings and closings.

So, instead of using the wrong tool for the job (a text parsing tool for a structured document) use the right tool for the job (an HTML parser for parsing HTML).