Umm yes, what happens if someone puts a few attributes in your parent node or the node you need and screws up your regex search as a result. Or a space? Regexs for XML will bite you on the bum when you least expect it. Parsing is slower, and can have its own issues, but is generally more predictable.
in reply to XML parsing vs regex
# your regex would fail if
<parentNode id="1234"> # would fail because node now has attributes
<parentNode > # just one space is all it takes
# or this...
<nodeINeed><!-- regex this comment, sucka! -->12345</nodeINeed>
If you spot any bugs in my solutions, it's because I've deliberately left them in as an exercise for the reader! :-)