Find names/words (from a string) that start and end with the same letter.

• Case-insensitive.
• If a name/word is not at the end of the string, it can be followed by a white-space or a comma.
• Names/words contain only letters or dashes.
• Underscores are NOT considered as letters. Words separated by underscores count as distinct words, e.g. in 'NAN_CONST' the 'NAN' is matched.
• Words are at least two letters long, so e.g. 'a' is not matched.

Example:

 in  = 'Cedric loves regular expressions'
 out = {'Cedric', 'regular'}