It told me that there was no 'foo' tag despite being clearly there and that being recognized as a 'foo' tag previously. Eventually I figured out that XML amespaces were to blame

I find it very unfortunate that XPath requires you to specify the namespace. I wish libxml had an option to configure what it meant to have no prefix in an XPath node test:

In reply to Re^3: parsing XML fragments (xml log files) with... a regex by ikegami
in thread parsing XML fragments (xml log files) with XML::Parser by kgoess

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