Potentially invalid emoji code (detect preceding alphanumeric character)

[87flowers]

What

The intention seems to be to detect if there are any alphanumerics on either side of an emoji code. (See #129.)
However, the code as it is currently written only detects the case for alphanumerics after an emoji code.

In other words, the current behaviour is: (以前の動作:)

  x:e:x       ⇒  TEXT('x:e:x')
   :e:x       ⇒  TEXT(':e:x')
  x:e:        ⇒  TEXT('x'), EMOJI_CODE('e')

We change this behaviour as so: (新しい動作:)

  x:e:x       ⇒  TEXT('x:e:x')
   :e:x       ⇒  TEXT(':e:x')
  x:e:        ⇒  TEXT('x:e:')

Why

Example note text that would trigger this:

8:30: Breakfast
9:00: Presentation 
12:30: Lunch

Additional info (optional)

We also extract notRegexpBefore as it is a pattern seen elsewhere in the code.

If this is considered "not a bug" I am happy for this to be closed.