regular expression and xpath are two approaches to identify a matching subset of content within a given amount of content for further usage. the first one based on plain text the second one based on xml.
both are powerful but if you not using it on a day by day basis are tricky and error prone. error prone means that you have to avoid all edge cases where a given expression shouldn't match but it does or the other way around where a given expression should match but it doesn't.
if you work in XML related world you often need both approaches e.g. using XSLT or XQuery.
there are few commercial IDE's which helping out to develop the right expression for the required use case but there are two which i'm often use dedicated to help you dealing either with regular expressions or xpath
powerful standalone tool to create and verify xpath expression. support for xpath 2.0 and most features you ask for in this context. this tool is based on well-known Saxon for xpath 2.0 and .net subsystem for xpath 1.0
powerful standalone tool to create and verify regular expression. with buildin regex analyser and on-the-fly validation, code generation (for .net and vb only), ....