Valid characters for actions, hooks and filters

1. When you “hook”/add_action/*_filter('whatever'); a callback function to do_action('whatever');, then you basically add the function (or object-method) name to the global $wp_filters-array.

2. Doing so, you add the function/method name to an array that is built like the following

   $wp_filter[ $tag ][ $priority ][ $idx ]
   // $tag = action/filter name
   // $priority = 3rd argument / execution order
   // $idx = "unique" name
   

3. Not the 3rd argument/$idx is built using the _wp_filter_build_unique_id() function, that takes the first args from add_action/*_filter().
4. Inside this function, every function name stays the same, only method names change. This is the reason, why you often find “funky” method names, prefixed with a pretty long number.

Can I add numbers after letters in do_action tag?

So yes, numbers are completely valid when naming action hooks. Summed up, it’s save to use a-zA-Z0-9_ as function/method/variable names. - is not supported. And while some characters might work well on your system, it mostly depends on the encoding if your chosen function name (example: _wUT?a_nice_DÄY!()) works or not. WordPress itself does not check if a function/var/hook/filter/whatever name is valid or not.

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Addition to @BrianFegter answer about “contextual hooks”.

You can see some “contextual hooks”, when you take a look at the »help«-panel in (any) screen in your (MU or single) installation, using this plugin¹⁾.

^{1) Plugin inspired by @StephenHarris article.}