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.
# 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! :-)