Regular expressions: whilst they are completely unreadable to the uninitiated, once you find your way in they are a pleasingly concise syntax capable of expressing very complex patterns.

In bash I use them all the time:

In C++ they can look a little clumsy, but they're massively improved with raw strings:

And they even turn up in Wireshark (right now I'm filtering XMPP traffic):

frame matches "from.*[[:alnum:]]+@localhost"

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