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Further topics (see ?regexp in R):

1. character classes: e.g.,

   1. POSIX names [:alnum:] [:alpha:] [:digit:] [:lower:] [:upper:] [:blank:]

   or

   2. sometimes shortcuts like \w or \s

   3. . (any character)

2. enumerators: *, +, ?, [1,10], [2,]

3. capturing matches and backreferencing, using () for capturing and $1 or \1 for backreferencing