See the question and my original answer on StackOverflow

The error you have is due to the fact that the expression re:test uses an XPATH function named test (declared in a namespace whose prefix is re), that is unknown to the XSLT context.

I don't know where you got that expression from, but it's not standard, so it means nothing in the Html Agility Pack context :-)

For indepth explanation, see this cool article here: Adding Custom Functions to XPath. Note you could make it work using these techniques.

That said, here a "pure" Html Agility Pack / XPATH implementation:

var pNodes = htmlDoc.DocumentNode.SelectNodes("//p[text()='=.+=']");

It uses a filter (between [ and ]) and the standard XPATH function text() which means "inner text".